That’s just semantics.
When you buy a CD, you don’t own the songs.
But you do have some item that belongs to you.
With Steam, you have a ticket that will let you into Steam to download the game for as long as your account is in good standing and as long as Steam exists.
With GOG, you have a file you can use to install the game on any machine INDEFINITELY. GOG can’t revoke your access for any reason, and if GOG shuts down, you can still install the games.
I have plenty drm free games in steam where I copied the game folder into other computers and it ran offline. At that point, there’s no difference between an installer and a compressed copy of that game. For reference, Grim dawn but there’s plenty more.
“Installing” is just semantics for decompressing a file in specific folders, you can the collect that data and “install” the game wherever. As long as you can run the game without steam, it doesn’t matter that you used steam to buy it.