Because those models are actually ridiculously common. The 512ke is in that strange place where it's definitely not "common", but it's not exactly "rare", either. Or, at least, it's not perceived to be rare. I mean, the IBM PC-XT isn't all that common, but it's not rare, either. It wasn't a "first of class", nor a particularly precedent-setting model, either. It's just kind of "blah". Not something that a non-collector would even really notice. Other than maybe "OOH! Is this the original? No? Oh, then it's probably not worth anything, therefore not really worth hawking on eBay."

(Note, choice of XT was because it's a vaguely similar model, timeline-to-its-own-line, the comparison falls apart quickly if you over-analyze it.)