Their DRM is optional, and many games on Steam are DRM free, meaning you can copy the files to another machine not running Steam and it would work just fine. And Steam's DRM, if used, is the least annoying DRM out there IMO.
Less games actually use Steam's DRM than people think. Even the ones that require Steam to run often just use their API for stuff like multiplayer functionality or displaying leaderboards.
There's an open source library that you can sub in to emulate the API and run the games on LAN without Steam. I believe there's no decryption involved so it should be 100% legal, just like how Proton reimplements Windows APIs.