Why isn't there an Amazon for skilled 1099 contractors?

By 'Amazon', I mean- an online platform where hiring companies and contractors list themselves, and can find each other. (Maybe I should've said Ebay). But- why hasn't such a thing been invented yet? I understand that several hiring marketplaces have been attempted for fulltime jobs (Hired.com, etc.) But that relies on getting the developer to actively sign up for the website, and they may not necessarily be thinking about a new job. Whereas, a 1099 contractor always needs to think about their next contract, after their current one is over.

So- why isn't there one? Platforms like Upwork and Freelancer.com (ugh) were invented in the late 90s, yet they don't seem to have really cornered the enterprise market. Why are 1099 contracts still being handled by 3rd party recruiting agencies? I'm not really interested in a typical HN-complaint fest about recruiters, I'm interested in understanding why a platform hasn't been a good business model (to date). Yes recruiters probably do a lot of non-scaleable/non codeable work in screening candidates, but that could be offloaded to an HR person at the hiring company. Yes enterprise sales is tough & annoying, but it's a huge moat once you're on the vendor list of every Fortune 500 company- hiring and compensating salesguys to sign up enterprise clients is what VC money is for. Yes, companies offload liability and whatnot with W2 contractors from a staffing agency- but that's not really the case for 1099ers, right? It's just a contract between the hiring company and the freelancer's LLC.

So- why do companies and 1099 contractors continue to use recruiting agencies to find each other? Isn't this exactly the kind of problem that tech can solve- that platforms exist for, where network effects leads to a winner-take-all market? There has to be a solid business reason why a 1099 Amazon or Ebay hasn't been invented yet, just curious what it is