It’s a common misunderstanding that the underside of a wing is flat and the top part curves. A paper airplane with thin flat wings still gets lift though there are several issues trying to scale this up. Similarly many aircraft will happily fly upside down.

Wings need to support the weight of your aircraft while being light this means they need to be reasonably thick especially using the obvious choice of storing fuel inside them. The first obvious choice is a teardrop shape which gets lift from being angled up similarly to the way a flat wing does.

Real wings don’t quite use a teardrop shape, but if you look at the front most part of a wing you see it curves both down and up. https://en.wikipedia.org/wiki/Angle_of_attack#/media/File:Ai...