POKIMANE HOT TUB STREAM
Not into armpits but since people are curious I am pretty sure that I know at least one reason that people are into them.

Especially if we take Pokimane as an example, it's a spot with very smooth skin, so similarly to a thighjob, I imagine that putting your ♥♥♥♥ under a woman's armpit or rubbing your tip against it could potentially feel pretty good.

Besides, someone could also have a thing for sweat which would explain people wanting to kiss and lick them.

It's a weird fetish for sure, but so is literally every fetish out there, so we really should not be judging each other for them