... Any reasons why KDE is so great, beyond its vast customizability?...
KDE is my favorite UI on any OS I've used, and that includes Windows, OSX, Android, etc.
I don't always use the additional KDE applications, and yes, I usually use the more mainstream ones like LibreOffice and Firefox. However many of those KDE apps are actually pretty good, and they provide reasonable alternatives which are nice to have.
It doesn't really matter though, because I use KDE primarily for the actual desktop environment itself. In additional to a really excellent desktop UI (launcher, taskbar, etc) it has so many minor utility applications that improve a Linux computer so much.
The fact that it offers lots of configuration options is really important for me as well.
I should mention that generally speaking, I install most of the other desktop environments as well, since many of them have useful utilities etc. In addition, it's also nice to have other ones in case you need to run with low resource usage or in the event that you break KDE somehow. (KDE doesn't easily break, but I hack around so much that I've often broken it by playing with experimental settings etc.)
So yeah, I love KDE. Thanks so much KDE developers!