It's not all about the hard work and training equaling reward.
Professors and science researchers probably work as hard as physicians, (ie: 4-6 years of grad school, 3-4 years of 'residency' while looking for tenure),for the same amount of time, in their schooling, and they get paid jack. However, it is free. If physicians education were free, wouldnt it be similar? Now i know you may say "physicians have more responsibility" , but, professors and researchers take responsibility for millions of research dollars, which represents a lot of hard work by thousands of people (the money invested), and they are responsible for the minds of the future generations, and the level of knowledge in our society (which, in my mind, is as important as our health).
The pay here is low, the responsibility high, and the effort expended and schooling endured is enormous! , yet i dont see any shortage of people looking to become professors and scientists.