It's the principle of "do one thing and do it well". There's nothing wrong with running games in a desktop but there are limitless ways of customizing a PC and it's impossible for developers to account for everything. It would be nice if you could just write some code and have it work flawlessly for everyone's setup but that's not how it goes. For the use case of the Steam Deck where you are dealing with a low-TDP gaming device it makes more sense to have something like gamescope which can just cut out all non-gaming processes entirely. Maximize performance and battery life with a nice interface to boot, and the desktop is still there if you need it. At the very least it makes troubleshooting super easy when stuff does go wrong because there's very few external things to factor in.
It's the principle of "do one thing and do it well". There's nothing wrong with running games in a desktop but there are limitless ways of customizing a PC and it's impossible for developers to account for everything. It would be nice if you could just write some code and have it work flawlessly for everyone's setup but that's not how it goes. For the use case of the Steam Deck where you are dealing with a low-TDP gaming device it makes more sense to have something like gamescope which can just cut out all non-gaming processes entirely. Maximize performance and battery life with a nice interface to boot, and the desktop is still there if you need it. At the very least it makes troubleshooting super easy when stuff does go wrong because there's very few external things to factor in.