The game studios have an option when publishing the game through them to use their DRM, there are a ton of games that allow you to run it with steam off or even uninstalled from the PC, it’s just a lot of games choose to include the steam drm on it (usually to allow for steam achievements as that is not possible without it to my knowledge), sadly steam does not provide a good way of identifying what games do this and what games don’t. I believe you may be able to check this by checking the games executable in the install location to see if it uses the steam schema