There are limits to the utility of doing anything better if you are already great. To ask why somebody doesn't do more of something presumes that it would be significantly beneficial, which it may not be.
Rather than spend my time learning yet another programming language, I am currently spending my free time: exercising, watching Lost, working on a personal project, and learning French. Spending several years of my life learning as much about programming as I could was very fruitful, but now I know enough that I want to spend most of my free time doing other things.
Sometimes you need to stop running, and just enjoy the scenery.