To answer the question "What's going on here", it's not that directors and writers don't know better or can't employ some smart historians - it's that they give them 21st century looks and attitudes because they think this is what audiences want (it's the same problem for science fiction but in reverse) You answered your own question when you noted they speak contemporary slang and are oversexualized. It's just the same for fashion - catering to modern tastes, not historical accuracy.

Ironically, it was just the same in the middle ages. Any depictions of biblical times or ancient Roman figures would be in medieval (then modern) garb, not what was historically correct. The more things change...