Clearly, different people are motivated by different things.
I would suspect that anybody motivated by money is more likely to go to Wall Street than become a founder of a start-up.
Not to say that founders are not in some way motivated by money, but I'd be surprised if that is the primary motivation for the majority. If you look at the most successful founders, I think you'll find they love what they do, and the money, once you are successful, becomes secondary.
I've started a few times, and have ideas for other start-ups (nothing overly successful yet). I don't think about 'what would make me a ton of money', I think with respect to what I want, what is the world going to look like in 10 years in an area that I'm interested in. I have a vision, and I research if it is a good opportunity, and I pursue that vision. Even when the signs are that it is a financially bad opportunity, I find myself motivated by the vision, and continue anyway.
Last year I ran a project (not a start-up, I never saw it as a business), and paid the server cost for a year, as an experiment to see if others agreed with my vision, if I could inspire people, if I could change some lives. It was a really interesting learning experience. It was successful at first, more than I thought, and then fell to pieces, but what fun!
The reason people describe founders as 'visionary' is quite simple, if you don't have a vision, I don't think you've got anything to start.