My guess is that some freelance hackers do indeed create startups, just not the majority of them.

Regarding development companies, well you'd have to ask yourself why the owner who has already spent time going through the pains of starting up, want to throw away a working business in favour of one that may or may not work.

The owner could encourage side projects if so minded, but it's more likely once their business is running, that they have too much to worry about running one business to have time to think about creating another.

Either that or they have lost interest once the dough starts coming in!