Why do emergency exits over the wings have "step up and through motion" rather than having floor level exits? My questions stems from the following paragraph from Wikipedia:


  Additionally, during aircraft evacuations, it has been found that the
  majority of overwing window exit designs of Boeing 737 (NG) Next
  Generation Line along with the Airbus A320, hamper evacuation in
  comparison with traditional floor level exits due to the inherent
  "step up and through motion" required of passengers as they exit the
  aircraft, unlike the designs of older generation wide body aircraft
  such as the McDonnell Douglas DC-10 and Lockheed L1011 which are all
  floor level.


One more question I had, wouldn't it be better to have the same emergency exit procedures with respect to opening the exit door etc. on all type of commercial aircrafts?
(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.