People want different things. The extra work added on different desktop environments is work that neither Microsoft or Apple is doing, and I think that's a good thing. If you want an environment for the average Joe, use desktop A. If you want a techy "fiddle all the knobs" one than use desktop B. If you want a minimalist one, use Desktop C, etc. MS doesn't make three or more versions of their OS suited for different populations, you get what they want to give you. Same for Apple.
Diversity is a strength of Linux. I don't think Pepsi worries about fragmentation with their various flavors of Pepsi, Mountain Dew, and whatever other hundreds of brands they own. Sure, diversity can be a problem if there aren't enough developers to go around but I'd rather have more options than fewer.
Same with billions of programming languages, frameworks.
Do you guys complain because there is more than 1 game studio? You'd rather have one movie/serie and a handful of actors director so all movies and series are made by the same team in order to not waste ressources? A single house architecture plan so that everyone live in the very same house with no customisation possible?
There is no one-size-fits-all solution here because there isn't a single entity controlling the development and access of Linux desktop environments. Pretty sure if all of those teams got together to somehow deliver a solution, neither the devs or the users will be happy with the result.
Sometime I wonder what can they pull off if KDE team, Gnome team, XFCE team and now Cosmic team got together.