Because its not just for 2/3 games but the entirety of GOG of which is ran by CD Projekt. Also GOG Galaxy is meant as an optional frontend since being DRM-free is GOG's schtick. The game's available on Steam and EGS anyway.
Unless if it's something else that I'm not aware about.
I get that, and I bought the game through steam. But when I launch it through steam, it launches the gog launcher, which from there I have to launch the game. Literally a game launcher launching another game launcher to launch the game. If you have GTA V on the epic games launcher this is also the case, the epic games launcher launches the rockstar launcher, and from there you can start the game. seems unnecessary