I'm not sure we're actually disagreeing. OfferLetter.io works precisely because you're focused on the negotiation part. There, your incentives are aligned (it's always in your interest and the candidate's to get them more money). And there, it makes sense to pay you because you're only paying if you do end up with a higher cash salary.

If you were to branch out into helping people find jobs, I don't think it would work. All of a sudden, you'd always be incentivized to find people cash-rich, equity-poor positions, largely limiting the pool. Money is important, but at the end of the day, it's just one of many factors. It just happens to be the most easily quantifiable one.

Moreover, if we ignore this disincentive for a moment, if you were to branch out into end-to-end job search (rather than just the negotiation part), you'd quickly become frustrated because for a lot more effort, i.e. everything before the negotiation, you'd be getting much less money than you would if you just struck a deal with the company side o the market instead.