>Why don't they make the skin out of radar transparent material with a complex internal structure?
>You could give the hull any shape required for optimal aerodynamics.
Diminishing returns + weight issues; but that IS done for certain edges.
If you look at the intakes you'll see the sharp edges are a different panel than the rest. Those are radar transparent, with an advantageous conductive shape below. So you get great aerodynamics and good stealth characteristics. This is also done on the wing and tail edges.
Look up "leading edge treatments" if you wanna know more.

This is actually one of the details the Su-57 lacks entirely, which is a big part of why its a total fucking joke stealth-wise.