A combination of reasons. I liked figuring out how things worked and creating new things, even as a kid. I was good at and enjoyed math and physics, but no so much that I specifically wanted to become a mathematician or physicist (part of that is that I enjoyed answering questions more than asking them). I liked the idea of a profession that paid well and had good prospects for advancement.
Also, I lacked the athletic skills to be a professional athlete, the musical talent to be a professional music, or the inherited wealth to sit back and do nothing with my life.