Because "negotiating" the blank check that you already signed does't work.

Yes, it works if they have actually stuck you with a bill that exceeds your net worth and expected income, but in that case you can't afford and/or don't need a negotiator -- you just default. It also works if you've stirred up a media frenzy, but you can't turn that into a business model for obvious reasons. The only place it does reliably work is in the imagination of those looking to downplay the severity of the current state of affairs.

They exist but many have actually gone out of business in the last few years, perhaps due to hospitals refusing negotiation. Most of mine were referrals from health insurance brokers. They're usually commission based so they only charge if you they save you money.

Like the parent comment said, there are. Insurance companies play that role. Hiring a negotiator without having to buy insurance would be valuable, though.