Sorta like Vanguard funds, if you're familiar with them.
Fees on these freelancer platforms are very high. In some cases, 99designs takes a 40% commission, for very little work.
I remember awhile back there was Scriptlance, which only cost $5 to post a job.
Now you might say this would only benefit freelancers. But if you think about it, the job providers would benefit too. If oDesk's commission on a contract is $500, that's $500 that could have gone to increase the payout. You'd be able to attract a better candidate or have the freelancer spend more time.
This wouldn't be for the money of course. And taking on the entrenched competition (with their network effects) would be hard.