The job of the entrepreneur is to build prototypes and quickly validate their ideas. That's what they get the big equity stake for. Once they can get funded, they can hire other people to productionize it.

Why would you do this instead of starting the company yourself? Lack of ideas?

I can think of a number of people that would interested in a service like this (search HN for "How do I find a technical cofounder?" or "I just need a programmer") However, from your perspective you're sorta getting a raw deal because you're doing most of the work involved in getting a company off the ground yet have little of the upside. And from their perspective, there will be knowledge transfer issues (if it catches on, you're the only one who knows how the software works), trust issues (how do they know you won't just steal their idea and run with it, like Zuckerburg?), and possibly money issues (idea guys who can't code themselves and don't know any technical cofounders tend not to have a huge amount of money to throw around).