Think about how many man-years have been wasted trying to "fix" the same problems—whether it's the KDE vs. GNOME divide, the battle between package managers (RPM, DEB, Flatpak, Snap...), or the scattered efforts to create a mobile Linux OS. Imagine the impact if even half of that energy had been pooled into a single, shared vision.
It’s great to have choices, but at what point does “choice” turn into unnecessary fragmentation?
UI development is an endless procession of nit picky long tail aesthetic and usability bugs.
The thing is that these issues are 100% real, I just don't really care to spend two days figuring out two labels don't align correctly, it's good enough.
Scaling this up to a desktop, then those small issues start to stack up. If you're then a Windows power user, then I can easily see Linux being a hard sell. I've seen plenty of people navigate Windows and Microsoft applications like they're some sort of wizard. I haven't tried KDE in a long time, nor have I given Cosmic a go, but if you don't just click around at a leisurely pace, then I have no trouble seeing that you could feel limited, compared to Windows.
Obviously I can see it opposite being true as well. I can't use Windows, something about the UI makes my hand cramp up (literally, there's something in the Windows UI that hurts my hand and wrist, it does not happen in macOS or anything Linux desktop I've used).
I honestly don't believe that to be the case. There's a big difference between commercial products like MacOS or Windows and Linux. That difference isn't so much funding (remember that Linux is backed by all major companies), but the fact that Linux itself is really just the kernel, whereas Windows and MacOS aim to be the whole package (with varying degrees of success).
There is simply no such thing as "Linux, the OS including desktop and UI". It's dozens of projects with different philosophies and goals. KDE differs from Gnome, Mate, or XFCE and those have a different approach to the desktop UI metaphor than say i3 or xmonad.
A standardized Linux Desktop experience would help with bundling resources as well as polish and fixing the "boring" stuff. But this would fly in the face of the "make-it-your-own"-approach that desktop Linux tends to take.
The constant dance between stability/maturity and bleeding edge is another big factor. The former allows for little annoyances to be ironed out, while the latter attracts more developers because that's the fun stuff to work on.
As a user you do have the choice to go for a stable, more conservative distro or play with the latest toys on rolling release distros. Specialized distros like SteamOS do quite well in their respective niche, though.
In summary, the Linux eco system is simply too diverse to point at a single issue like funding as the main reason for its perceived shortcomings on the Desktop side of things. The comment above pretty much nailed it - it's developed by Linux guys and their goal simply isn't to emulate Windows.
I picked an unimaginative Dell (my house was burglarized right before I started and I lost my then daily driver) because it had the shortest delivery time.