That's interesting. Isn't one of your primary benefits (from the employer point of view) the fact that you are a premium supplier of great talent? "Great talent" including "reliable"? If not, what separates you from any other freelancer website?
Nonetheless, renting a startup or (team for that matter) is still a new concept for employers and they are used to paying an hourly rate for consulting gigs. If we start to see way more demand for flat-rate project from startups, we'll move in that direction.