The airbags inside the car are meant to cushion the hard surfaces of course and the curtain bags are meant to keep people and their limbs in the protective passenger area when the car spins, but also they are acting as a counterforce and slow the speed of the person a little more gently, versus an instant stop to a solid object such as the dashboard, causing internal injuries as well as the external trauma.
Also airbags are a very good reason to wear a seatbelt - if you pop the airbags while colliding at relatively slow speeds, you'll probably lean forward a bit from the deceleration only to be punched in the face by the bag (and probably get some vertebrae renovated).
So while installing external airbags to cushion the pedestrian impact sounds like a good idea it probably isn't, as the bag suddenly expanding from the direction of the vehicle would simply add to the energy and "punch" the pedestrian away from the car at the 200mph at which an airbag expands.
Lastly, I'm all in for the gel from Demolition Man, why aren't we funding it?? Would it come in different scents and colors? :-D Really the car could one day - when detecting an unavoidable collision - spray out a foam or expanding gel that would cushion the surfaces and also decelerate the fall of the pedestrian - I'm thinking Final Fantasy Spirits Within -soldiers drop gel"?