I am aware the DE can be changed, but it was just an honest answer to OP's question. I downloaded like 8 different distros and put them on flash drives and tried them all out and that was what caused me to move on. I didn't have kubuntu downloaded to try, probably because canonical seems to treat them as entirely different distros.

ie, some distros have the DE options when looking at the download page or have you choose during the live boot which to use and include multiple in one iso. Ubuntu makes no mention of those separate downloads unless you explore their site a bit further than the download page. It's a minor difference but makes a difference when you're grabbing a handful of isos to try out, you might miss it and assume the one iso has all the options available when it doesn't, or that it is the only option they provide.

As for PopOS I actually did look into changing to KDE and the popular wisdom at the time on message boards was that changing to KDE would possibly or likely undo most of the benefits of the tweaks and changes system 76 made. I don't have any idea if that is even true, just what came up when searching a few years back.