Might depend where you are? They’re readily available here, for example, and the staff will install them free (if they’re not too busy).
Having said that, I’ve noticed that on some cars, changing the entire wiper assembly is pretty straightforward so would be easier and less hassle than just changing the inserts.
I remember regularly buying the inserts. Never had a problem. Then the parts stores quit selling them, except for “one size fits all” that you have to cut to size. And they’re shit. My car dealer’s parts counter sells refill inserts but charges more than it costs at the parts store to replace the whole blade. So that’s what I do, and it has the benefit of being better than what came on the car.
On my car (2010 Subaru Outback) changing the front two wipers takes 5 minutes, tops. The rear wiper is a PITA but I change it much less frequently. So swapping the whole wiper is quick and painless.
It’s so wasteful. It’s shamefully wasteful for a small amount of convenience.
That’s basically what I figured. It doesn’t take much of a mistake to ruin a very expensive windshield. I do remember installing some inserts back in the 1990s and with the triple-edge blades it was a bit fiddly to get the clips into the right groove. If you didn’t get it in just right then it would streak badly.
I do know the auto parts stores generally don’t stock the longer-lasting silicone wipers for the exact reason you’d think, but they still sell them online. I got some PIAA wipers in October 2019 and had to order them from AutoZone’s online store. They’re still going strong almost two years later and I wash/wipe my windshield a lot.
Where the hell do you live?
Replacing whole wiper blades instead of having inserts is an insignificant amount of waste compared to having personal vehicles and the accompanying infrastructure instead of extensive public transportation.
My guess would be Calgary.
Amazon sells wiper blade inserts for less money than whole blades. Unless you’re pretty good with working with such things (and enjoy it like I do) I wouldn’t recommend trying it. The OEM blades aren’t made with replacing the inserts in mind these days.
One big win with using inserts was with the rear wiper on my hatchbacks. With a bit of finessing, I used part of an old but still serviceable blade’s rubber to replace the rubber in the rear wiper. Parts stores want crazy prices for rear wiper blades!
I was going to guess Winnipeg, but Calgary works, too. 
Well, there’s only one of those that I have full control over.
Also, it’s a false dichotomy. I can try to reduce all of my waste in these incidental ways, while supporting the kinds of infrastructure changes that would allow me to not have a personal vehicle.
But no, right now I could not give up my personal vehicle. I do telework (almost all the time at the moment, but that will go back to 1-2x per week at some point) and carpool to reduce the impact of my commute.
NE Alberta. You don’t have to be insane to live here but it sure helps…
I’m guessing they just didn’t sell well enough to continue to use the shelf space on them. Put another way, Wal-Mart or AutoZone or whoever decided that the sales of inserts didn’t justify carrying them versus something else for that shelf space that they might make more money on.
I think but don’t have any proof, that the wipers used to be worse back in the day; I remember changing the inserts a lot more often in the 1990s than I do the blade assemblies these days. I remember changing the inserts 3-4 times a year, while now I think I change my blades maybe every 9 months or so, whenever they start streaking and can’t be remedied by cleaning the rubber part off.