Well yeah, but that’s what standards are for. Look at Wayland. Outside of GNOME being a bit slow, all the major compositors and DEs like KDE and Hyprland have agreed to implement certain common desktop features that every desktop should have along with the Wayland protocol itself. Then they go their own way.
So it’s not really as you say. There is unity in development beneath the heavy diversity.
But along the way, things that would be considered “requirements so basic we don’t even need to state them” are not met.
Except they are
Your desktop doesn’t have a cohesive look with colours and fonts mismatched in a way that no monolithic project would ever tolerate.
This is a bad example bc it’s opposite to what you say. You haven’t set a universal theme. Theming is commonly supported across desktops to some degree. You can get all your apps to have the same look unless an application forces its own (which would happen even in a homogenous Linux world). Some desktops will do it for you, but if they don’t it’s still as simple as install a universal theme, apply the Gtk version, and apply the Qt version. My understanding is soon it will be even simpler once a few more Wayland standards get adopted.
We already have the best ecosystem. It literally could not improve functionally