You assume incorrectly. I am not rich by the standards of rich people (although I am rich by the standards of many others, but I think it's safe to say that most of the people reading my comment right now are wealthier than me), and I don't form startups because of a desire to gain wealth. I have no actual interest in amassing wealth as such.
I form startups because they give me the opportunity and freedom to engage in projects that deeply interest me.