That's an oversimplification.

For starters it depends on the game and there is zero information in Steam about whether that works or not for a game before you buy, so you can't make an informed buy there if you care about that.

Further, for many games you have to install something like the Goldberg Emulator which is an implementation of the Steam API DLL that doesn't in fact talk to Steam.

Without this some games will stop working because the Steam application is not present or the game has been started offline too often.

Even then it's not guaranteed to work, especially if you have DLCs for that game.

Disconnecting an installed game from Steam or backing up and later using a cached installer is a technically more demanding process and with a higher rate of failure than just downloading and backing up a GOG game's installer that you later use for installing the game - it's actually GOG's Unique Value Proposition that you're guaranteed to at a later date you're still able to install a the game in a system compatible with the installer (in my experience only a problem when trying to run an installer over a decade old in a more recent OS, but emphasized because even without phone-home related limitations, there are still possible hurdles due to evolving technology and insufficient backwards compatibility) so it makes sense that their process is seamless and works for all products.