Gnome doesn't want thousands of developers implementing their own visions for their projects and others don't want to donate their time to implementing gnomes vision.
The proper thing for them all to do is all implement their own visions and people to use or work on what they please.
Nobody asks why Tesla, Ford, and Toyota are wasting everyone's resources by being different company. Nobody suggests having different car manufacturers is paralyzing. Nobody opines that once the perfect car is invented we can just deprecate the rest because those would all be silly positions.
The car market and computer uis are an evolving multidimensional entities whose evolving product lines are and have been dependant upon many parties pulling in different directions.
It's like looking at only a duck and a pond and asking why the universe couldn't have just made a duck because it would be an ideal choice for that place and time. Of course it couldn't possibly work like that because arriving at that exact solution without intermediate steps would be impossible and besides the duck is no good in the desert or tundra.
Also gnome is so flawed in so many ways from leaking memory due to unfixable mismatch between js and compiled code, to nonsensical handling of multiple desktops, to add-ons that both rely on monkey patching your desktop due to lack of addon api and can with a single crash kill your whole session, to hostility towards themeing, to ugly header bars, to hostility towards support for non gnome desktops.
It is a worst in class solution.