Why do emergency exits over the wings have "step up and through motion" rather than having floor level exits?
(Image source)

The reason is the fillets (wing root fairings). They're curved as shown above.

You could have a plane without those fillets, but then there are two downsides:


Considerable increase in fuel consumption, and thus reduced range
Potential extra strengthening required for the fuselage/wing connections, as a direct floor-to-wing-top exit might change how the load transfers are distributed, resulting in increased aircraft weight.