Do people forget that games used to require you to have the CD-ROM in the drive before they would run?
They weren't always like that though. Don't forget piracy didn't start with the video game industry. It only started once it took off. CDs came later.
Source: person who remembers playing games off 8" floppies.
Edit to add: a game 20 years ago will only run because Windows says it's ok. If it's a linux-based game from 20 years ago, then it depends on a lot of other stuff. It's not Steam that keeps them running. Steam just provides you a copy for the most part. GOG exists and doesn't have the DRM that Steam allows. Does it have the same library? No. But we shouldn't support DRM to begin with, so if it's not on GOG, than I don't trust the game itself.