I think this aspect of the game has not been adequately explained yet. There are a lot of ways to play Magic, called formats, and different formats involve different subsets of the total universe of cards. (There are tens of thousands of different cards.)
The most common format for tournaments and events is called Standard. Roughly, Standard consists of the past couple years worth of Magic sets. (There is usually a new set every quarter. Some sets are standalone, some are part of a larger story or aesthetic with other sets.)
So every few months, the oldest set(s) in Standard drop out and new ones are added. Thus, if you want to compete in official tournaments, you will have to regularly buy and trade for new cards to make new decks that conform to whatever is in Standard at that moment.
That doesn’t mean that your old cards become worthless, because there are other formats in which they might still be useful. Someone above mentioned Modern, which is non-rotating and consists of everything from 2003 (8th edition) forward. There are indeed some crazy expensive Modern decks, mostly thanks to fetchlands, which WOTC is not going to print any more of. (Not because they want to keep the value up, but because they are a huge time-waster in tournament play and they don’t want to reintroduce them to Standard.) But there are budget Modern decks that can do very well if you play them correctly.
Another format is Commander, also called EDH. This format limits you to one copy of every card (except lands) making deck building a very different process than other formats. You can also use pretty much any card from Magic’s history. There just aren’t that many super-expensive cards that also have great synergy with one another, so Commander decks are often fairly cheap, often relying on cards that were somewhat under appreciated in their time. (WOTC also sells new premade Commander decks every year. Some of them are good.)
Each format has different lists of banned and restricted cards. Cards are generally only banned outright when it turns out that they completely break the game. Cards that are banned in one format may be playable in another.
Of course, all of this is moot if you’re just playing at the kitchen table with friends. You can play with whatever cards you like, so long as everyone agrees. And go ahead and laser-print some fakes if you want.
My favorite way to play in person with friends is Cube draft. This involves one person assembling a large “cube” of cards (typically 360 or 720 cards). Each player then takes turns drafting cards and trying to put together a good deck from what they get. This requires zero ongoing expense once you assemble your cube.
Another popular casual format is pauper. As the name implies, it restricts the game to only common cards. You can often put together a pauper deck for under ten bucks.