The reason is because it doesn't work. How do you walk forward continuously for a long stretch? Do you roll it to some position and keep it there? Do you keep rolling?
Granted, it doesn't reach console levels of use on most days, but I only clean it once every month or so, and even then it's easy - Logitech has a simple system to pop the ball out and cleaning it is very easy and fast.
Using a trackball is definitely not a major chore.