They technically have defaults, but when the difference for most distributions (e.g. Debian) is just checking a different box in the installer, it seems like a silly thing to complain about.
I think there's an assumption that the default DE gets more attention from the distro; it's more polished, and "just works" (or at least, something approaching the concept on Linux).
e.g. from what I remember, when installing xfce in debian, you get the ugly barebones defaults from upstream. Compare this to Xubuntu...
I made the switch from Windows 10 to Manjaro w/KDE about 2 months ago. Took about a week of ironing out some things and forcing myself to stick it out, but now I love it. I will also vouch for KDE being really good right now.
