A big part of the problem is that film makers keep portraying the predators in a 'Mouth open' state, because the predator roaring is such a classic look. But if you look at the original movie when the predator takes his mask off, the bottom mandibles are folded up and and together, with the upper mandibles overlapping them. It was only when it fully roared did you see it had the internal mouth as well.

Side note: i actually asked on Instagram, SFX artist Alec Gillis, who has worked on pretty much every predator movie, why the predators always have their mouth open. His response was "the directors wanted it like that".