Steam is just a storefront, not a publisher. And traditionally Valve only publishes their own games. While Nintendo, Microsoft and Sony are more broad game publishers. Imagine Casio trying to charge the watch store for every time you look at the time in the watch you bought there. Complete bonkers.
It shines in 2D where Unity falters, yes. But it’s perfectly capable of doing 3D competently. It’s shaders and lightning pipelines that are a bit rough on the edges, but that can be overcome with time with more brainpower coming in to contribute. The scripting is also far more robust than the hodgepodge that Unity tries to pass off as C#. The great advantage is that Godot is a non-profit foundation with a transparent governance model. Not a predatory venture capitalist behemoth like Unity.