I especially see this with video games. A lot of my friends own thousands of games, many of which are either still in the shrink wrap, or that they have beaten and forgotten about years ago. Why do people do this?
You can see this in any hobby where you need to buy stuff to do what you “want” to do, like boat sailing or rock climbing.
Some people are more “in love” of the idea doing the thing than they are of actually bothering to do the thing.
Hence the RV that rarely leaves the driveway or the 40 foot sailboat that rarely leaves the dock (much less actually sails from New Orleans to Belize even once).
Because “having” is more important than using to them. I don’t really get this mentality, as I like to keep my stuff neat and trim, but they get their happiness out of the act of buying. The problem I have seen with that is the only way to get more happiness is to buy more. Not prudent!
Happiness comes from within.
With video games, the physical copies can also be considered collector’s items. So even if they no longer wish to replay that particular game, it might be part of a larger set. Also, if we’re mentioning virtual, Humble Bundle has made the gigantic Steam library a thing. By offering a low-priced pack of 10 games, of which you only want 2, you end up with all 10 because it’s still a deal.
And billfish678 has it right that many people like the idea of something more than actually doing something, too.
I recall hearing a stat that something like 80% of physical movie copies are never watched. I’m unsure if I’m recalling the number correctly or if it’s even accurate, but it’s not so far outside of the realm of possibility. I have several movies I’ve bought and never watched, and there’s several reasons for that. Some of them are, I intended to watch them, but I just never got to it. Or maybe it’s a movie I enjoyed and want to own, either because I want to be able to watch it when I want, whether or not that situation ever actually comes up, or I just liked it enough I want to pay more money toward it and own a copy.
With music, which is far more important to me, besides that I make a point of listening to every album I own, there’s also a lot more opportunities to do so. A movie I have to set aside a couple hours in front of the TV. With an album, I can listen to it almost any time. I sometimes find one that slipped through the cracks, but that’s rare.
Games can be worse. Modern games are such a time sync, that one can buy a game with the hope of playing it and just never having the time for it. Or they start it, get bored with the intro section and give up. But really, with a lot of games, selling them used just isn’t worth it. You buy it at $50-60, and they’ll give you like $5 for it when you’re done. So, really, if it’s a game I liked, if I’m going to only get a few bucks, maybe I’ll want to play it again later, maybe even years down the line. Hell, I play my old SNES, N64, etc. more than my modern stuff these days. Further, many game companies are trying hard to just completely kill the used game market, so we may not even be able to sell stuff used anymore without having to post on eBay or Craigslist, which many people find to be a pain.
I think some people get a thrill out of buying things, a thrill that diminishes when it comes home. I know of someone who had 3 DVD players in their garage, never opened.
But there is a time budget versus money budget issue also. The amount something costs is clearly marked on it - the amount of time you have to devote to it is usually not. Look at something like those 50 worst sf movies ever made packages consisting of stuff that fell out of copyright. You can pick one up for $20, affordable by almost anyone - but it would take about 75 hours or more to watch the damn thing. Same for video games. You can pick up a few in one hand, but playing them could take weeks.
I wonder if those of us who have obligations are still working in college or high school student mode, when we had lots of free time.
A friend of mine frequently visits thrift shops. He buys items that he believes have some resale value. And plans to sell them online.
The problem is he never attempts to sell them. His place is full of these bargains.
Me. I buy way more cosmetics and skin care products than I use. I love the stuff and I love buying it. I just don’t get around to wearing it all that much.
Books. I also buy too many books and buy them faster than I can read them. At the current rate, I think I need to live to 150 just to get through the current pile.
I buy things when they’re “a good deal,” even if I don’t want them that much, so they often end up forgotten. I know it doesn’t make any sense.
Because you can easily buy the materials needed to do something, but you can’t buy the time to do it in.
That’s keeping something you might come back to one day, not buying something you never use. I think it’s pretty normal not to beat a game then get rid of it the next day.
I think about this pretty much every time I walk the dog, I swear. The majority of houses in my neighborhood have their garages packed with crap the owners have (clearly) not touched in years/decades, and the cars parked in the driveway or on the street. I am not a clutter person, so this bothers me.
Time and/or energy. I’m certainly guilty of this. I know I have wood in the garage that will never be used in a project. If I quit my job today and became a hermit (honestly, not that much of a leap) who did nothing but woodworking, I’d probably run out of material in a few years. But lately, a box or bowl is about all I have ambition for, and my plans for drawers and chests come to naught. But I still buy tools for woodworking, partly in the hopes that it will inspire me, and partly out of habit. And, if the price is right, a nice piece of figured maple or cherry will somehow end up in the back of my station wagon without my having any specific use for it.
I think part of the reason for this is that one tends to identify oneself as a practitioner in a given hobby even after that’s no longer strictly true, or after other hobbies have superseded it.
TL;DR I’m going to make a lot of woodworkers very happy at my estate sale.
It’s also true that when I threw out all my VHS tapes a couple of years ago, many of them had never been watched. Sorry, Mrs. Parker and the Vicious Circle 
I think it’s also partly because some people are buying the dream that they are X type of person. If they buy a bunch of games, they can picture in their minds that they are a gamer and are up on the current games, even if they never get around to playing those games. Or if they have a lot of tools and various supplies for projects, they are a handy, DIY type person, even if they only work on a project once every three years. That person could sell all their tools and clear up some space, and then just borrow or rent tools that one time every three years that they actually need some tools, but then that would be admitting to themselves and everyone else that they aren’t actually a handy type, they just like to pretend to be. Or the gamer could get rid of all those games that they bought, but then that would be realizing that they don’t have all the free time that they used to have, and are now much more busy with adult responsibilities, and how they’re not exactly thrilled with that.
This is a big topic of discussion on guitar message boards. How many is too many? When some dude links to pics of his guitar room, lined with dozens of guitars - sigh. Other folks like to show how amazing it is that they get by with one guitar when they’d like a bunch. A perennial thread: “Are you a “player” or a “collector” dude?!” Ah, the testosterone.
If my Toy Budget isn’t concentrated into the fewest, best examples I can use, I am not maximizing it 
So true. Personally, I consistently overestimate how much free time I truly have to pursue interests, and wind up with a bunch of unused and lightly-used stuff in closets as a result.
My excuse is that I’m a collector. I have most sf magazines since 1950 and most DAW books up to number 300. I’ll actually read them eventually. But I have cut down at least.
My favorite is when it’s a room with like forty Telecasters in the same pickup configuration but different finishes, and dude is vehement that he needs them all because they sound so different.
I get a sense of accomplishment out of buying. If you don’t have the time or energy to actually accomplish something. Shopping for it is the next best thing.
It’s a good thing I don’t like stuff.
There’s also the phenomenon of people actually liking the activity, but due to various reasons (geography, time, health, etc…) rarely get the chance to engage in it.
I know that for whatever reason, the things that I love to do, but do infrequently, are the ones that seem to accumulate the most crap vs. the ones that I love to do and can do when I want. I think it’s because you can browse the web and/or catalogs and buy stuff and feel like you’re closer to the activity, even though you can’t really engage in it as often as you’d like.
There’s also the gadget-dork hobbyist phenomenon that takes place in every technically oriented hobby. The one that comes to mind is in cycling. I remember when I was into mountain biking, I’d meet weight-weenies who’d obsess over trimming every ounce of weight off their bikes, and yet still lug around 25 extra lbs of gut. It wasn’t the performance they were really concerned with, it was optimizing the bike components.