Aside from the fact that logging into every game separately would be a nightmare, it would only work for online games and be a major hassle for developers because it means they also need to compensate for not having a launcher on things like automatic updates and deployments. It's not really a solution either side would like.

I'm getting downvoted hard but people are forgetting that a game store not having a launcher is suicide. GOG tried that, started bleeding money, caved in and made their own launcher. Steam also has 20 years under their belt so saying worse launchers shouldn't be allowed to exist would just kill competition entirely.