The reason for this is Racers want the thinest tires possible for the rotating weight of the tire has more affect on one's ability to peddle then the same weight on the frame. Thus the lighter the tire the easier it is to peddle and the faster one can go. The down side of this is most bike tires are just to thin, which makes them susceptible to any road hazard.
Given this situation I always add an Tire liner between the tire and the inner tube. I find these make the tire "Thicker" so that the tire can better withstand road hazards. I have also used inner tubes that were thicker on the bottom, but I have found the Tire Liners much more effective.
Everyone gets flats, the real issue is how to minimize them. I have found the Tire liners that makes the tire "Thicker" is the best solution.
Some examples of Tire Liners (I have seen better ones at most bike stores):
http://www.bicycletires.com/tek9.asp?pg=products&grp=93Please do NOT confuse these with "Tire Tape" or "Tire InsertS" which are to protect the tube from the Wheel rim. The tire liners are flexible pieces of Rubber that lay in the tire. This adds an additional barrier between road hazards and the inner tube.