Because of the second "M" in the term, i.e. "Multiplayer". MMOs are as much about the community as the game. For some they're more important than the game. MMOs generally only get shut down when almost all of the community has left... or in other words when a lot of the game's "content" is no longer available anyway.

That's not the case for singleplayer games, or games designed for local/couch multiplayer. There's no natural limit on the game being playable. For PS2/Xbox era stuff and before, as long as you have game and the hardware you can fire it up today and have the same experience (not counting your worsening reflexes). There's no justifiable reason why a company should be allowed to decide that you can no longer play the game you bought on hardware you physically possess. The only reason is greed, pure and simple. That's why people aren't okay with it.