Because if you want professional game developers to exist then you have to be welcoming to them when they are just aspiring game developers. Kids who play lots of games and want to have a fiddle around with tools for making games are much more likely to do so if there is a way to access them that they are familiar with and already associate with gaming.
I don't think anyone is arguing that because there is an option to install via Steam that people should stop downloading directly from Godot's website though. Both of those things can exist beside one another.
godot doesn't break stuff in minor releases, and steam version of Godot has separate release tracks for each version (you can switch between godot 3 and 4)