Why? It's his choice. He could easily have cashed in to the tune of millions per year. Through public speaking, endorsement deals, etc. He chose not to. His choice, his reasons.
He seems ultimately very comfortable with the way he is living, feel free to prove me wrong but I do not remember seeing any point where he expresses a desire for more money.
I think this community as a whole at times, sees the end goal of money too much as the incentive and can often forget that it is mostly about keeping your head above water, and if doing so you manage to be happy doing what you are doing then that is a massive advantage.
Luckily for the vast majority of us who work in tech, our jobs could equally be said to be our hobbies, I frequently come home from a day of programming to start into a side project. I would much rather work on the wage I'm currently earning, than earn twice as much working in a different field where I cannot wait for the weekend and every morning seems like a struggle.
The gap is quite apparent in many of the tech pioneers life. For example, the late Dennis Ritchie was a pioneer for many, many technologies that I, and many other engineers, used on a daily basis for generating income. This gap for Dennis is enormous!
