:') Probably has something to do with the unscientific literary devices, like Tarzan being struck in some otherwise unknown gland and caused to shrink down ("Tarzan and the Ant Men" or something like that). Another example, how time was, uh, relative, in Pellucidar because the sun was always at high noon. So, two people could be walking, both get tired, and both lay down to sleep. One gets up after a while to tinkle, and when he gets back, the other person who had been sleeping has been grabbed by slavers and spent six months as a captive.
Farmer has no idea apparently that Tarzan is widely recognized because of the many movies, not because of the books, which hardly anyone nowadays reads. :') Burroughs isn't what I'd call my favorite writer in any genre. But I enjoyed some of his books, including some of the Mars stuff, Pellucidar, and that one about the hidden island in the pacific somewhere that was still home to dinosaurs.
Preferred R.E. Howard's "Conan" though. (':