Thinking about some disintermediations. On the supply side with consumer goods you have replicable stock (buy in more t-shirts/potatos). Presumably this is not how jobs work. You need someone to go around harvesting individual opportunities. Like collecting truffles as opposed to growing potatos.
With other unique items such as houses / ebay on the demand side you have various market entry costs: can the 'buyer' physically come over and see the house / can you get their paypal details? With jobs it's very easy (cheap) to apply, while for the employer it is costly to assess applications - making the pre-screening service offered by recruiters attractive.
Plus to disintermediate you'd need employers to directly deal with more of the HR process. That's open to them already, I suspect it's just not something people relish. Like you could save money washing your car but often it's just seen as a pain to be avoided.
Perhaps there are other improvements besides disintermediation: e.g. jobs sites with recruiter reputation/feedback. Seems pretty obvious but I actually haven't seen it around much. Can imagine there would be some bias (everyone who didn't get a post is peeved, plus some that did).