Exactly, so why isn't there more hair on places that need mechanical lubrication other than the armpits?

What places?

The creases between the tops of the thighs and the abdomen, the crease between the butt cheeks.

The angle would not be right, you want hair growing slightly off an roll parallel where it should lubricate. Armpit is bit of problem because it normally goes 180 deg so they cannot choose preferential direction. There the optimum is probably more radial then linear.