They don't? Ever heard of "bush planes"? They operate almost exclusively off very rough undeveloped fields using "tundra tires" among ather things.
http://en.wikipedia.org/wiki/Tundra_tires
There are a number of planes that have been adopted for "bush" usage.

Bumpy and heavy/long grass? They would consider that smooth in many cases. Ever see a plane take off out of a corn field with 4 ft tall corn stalks in it? I have. Not a normal proceedure but 2 days after an emergency landing due to smoke in the plane.