You have to take an already known scammer's word for it. That's not any assurance at all.
Hence the need for a third party. Once a third party can vouch, the value of maintaining positive status with them exceeds that of not returning access after the ransom is paid.
Think of the amount of effort that goes into gaining control of one of these systems. The win rate of actually getting paid the ransom is probably abysmally small since, as you said, there’s no reason to trust someone you already know is a hacker.
If that third party “plays fair” rating got the payout up to even 30% it’d be worth it to stay in their good graces.
On the other side of the equation, some of the legitimate companies whose systems get compromised probably would be willing to pay a ransom, if they had some reasonable assurance it’s only a one time hit and they get access back.