When you “add a game” to the steam library, you’re just creating a link to another file on your system, not really shifting the management of it over to steam (so no updates or the like), and if you logged in on another machine you wouldn’t be able to download the game through steam.

more importantly you can’t take a steam game and move over your license to use it, or ability to install/update it to some other platform. If you decided you never wanted to use steam again, that you liked some other platform better, you would still have to use steam to access any games you purchased there.

Edit: just an after thought to clarify my thinking on this. You payed to accesses that code. That series of instructions to be run on your computer. Everyone who worked to make it has been payed. If they don’t have money to keep maintaining it, they should stop doing that, or ask for further money to keep doing so. But if you want to just run the code you paid for already, it is absurd that someone restrict in what way you run a series of commands on your computer. It is indefensible, and corrosive to society.