They are notoriously hard to get right, require lots of work invested, and easy to break (regressions!) but are not too sexy or interesting, require insane amounts of QA and don't have low hanging fruits. As a consequence, nobody wants to do them, and so the Linux desktop will probably always be a mess.
Oh, and then there is that aspect of it that unixes in general were not built with the desktop in mind, so things like vm.swappiness defaults might be completely wrong (Ubuntu still defaults to 60 to this day!) and the kernel low memory handling is just outrageously bad, if you compare it to Windows, for instance.
I would be the first person to pay proper money for any Linux-based OS that at least _attempts_ to solve these issues. I might even go as far to purchase a laptop specifically to get the OS on that laptop, like the Macs, however I'm not willing to sacrifice on the size and quality of said laptop, actually, I expect a very high quality product for my extra money, so unfortunately at the moment there is nothing out there right now that I'd buy based on this.