Developers are highly paid because their knowledge is highly leveraged. Someone else is benefiting from your application of knowledge by an order of magnitude of what they're paying you. Why be the cow when you could be the dairyman?
To put that in non-cow terms, the rewards of just getting straightforward compensation these days are really good. So good it's not obvious that starting your own thing is worth it monetarily.
Chasing (relative) peanuts is still a better call for most.
See, I don't think most people care if they are millionaires or billionaires. And you can become a millionaire on a software dev salary.
Because the cow doesn't need to worry about selling the milk, collecting money from the customers and fixing the roof on the barn?