A combination of reasons.
I have a strong dislike for big government. I'd love to someday have the ability to defend people who are screwed by bureacracy.
But I put myself first... Can't help others until I am myself free of working for others.
In my current career, I will be a wage slave forever, no matter what my pay scale.
A genuine intellectual interest.
A love of what money can bring me.
A desire to put my kids in the schools I could have gotten into, but could not afford.
I want the ability to retire in style to a little college town, study ancient history, archeology, physics and paranormal psychology, take trips to interesting places, and enjoy my Senior years as the most rich of my life. The idea of retiring on a budget is boring.
I will never be the next Bill Gates. It is at least theoretically possible that I could be the next Gerry Spence.