It depends on the game and how they handle steam, if they see steam as a requirement then the game is choosing to use steam as a very rudimentary (and easily bypassed) DRM. But this is more about lazy development than DRM, essentially they're not expecting the steam APIs to fail, which is ridiculous considering they have non-steam versions, so a simple if statement would solve this issue. Also this paints those games in a very bad light to me, because if they're doing that with some API call on steam they might be doing it with another and now the game needs to be online always.

There are plenty of multiplayer games that don't require steam, iirc all of the paradox games you can just copy the folder to a different computer without steam and run the binaries.

And while not ideal, someone else pointed in another comment that there's an open source implementation of the steam API, so worst case scenario you just replace the library in your backup and you're done.