Why are Bananas Bent?

To answer the question I'll need to explain some things about plants.
Plant roots are positively geotropic meaning they grow with gravity down, they can sense earths gravitational field, stems are negatively geotropic meaning they grow against gravity up.

They do this because of the auxin a plant hormone.
Evolution plays a roll because bananas are actually stems that have been modified to a banana. Modified stems include also bulbs tubers and corms.
So bananas start out life very straight but pointing down - because they are actually modified stems and stems are all negatively geotropic as they grow and spread out they grow opposite to earth gravity bending them.