They do it because if you have to be online, connected to their servers, you have to look at their store and be tempted to buy something else for the game. It’s also just straight DRM. The industry spent the better part of 20 years complaining about piracy and used game sales, and now they’ve found a way to defeat them by just designing their games to disappear when the servers are gone. That does come with a catch though. Building and maintaining the online infrastructure costs a lot of money, and given how many of these games just instantly flop and die, customers are less willing to invest their time and money into a game unless they know it’s a winner, which has less to do with the game’s quality and more of how many other people perceive it to be quality. This looks to me to be why the industry is crashing right now.

As egregious as horse armor was decades ago, that doesn’t offend me the way server requirements do (you can always just choose not to buy the horse armor and still have the game you bought in perpetuity). If the game requires an online connection, don’t buy it. There’s always another game out there like it without the requirement. A game that requires an internet connection is just a worse version of a game they could have sold you without it, and the online requirement gives it an expiration date. If multiplayer requires an online connection, make sure it supports LAN, split-screen, direct IP connections, or private servers. This information is very hard to find just by store pages, perhaps intentionally so, but I usually check on the PC Gaming Wiki these days; otherwise you have to hope the developer responds to a question about those features in the Steam forums.