Lol okBiomimetically, it's because humans are use to holes that we insert stuff into being at the bottom.
It's actually a little more complicated than that...Few of reasons why you might make that design decision.
Aesthetically, bottom of phone is where you hand is holding it/covering much of it. Hide the ugly stuff like charging holes there.
Practically, it's because the top of the phone holds a lot more electronics (cameras, front&back, speaker etc), so a charger up there would be a waste of space.
Biomimetically, it's because humans are use to holes that we insert stuff into being at the bottom.