Ransomware relies on trust, because if you don't actually hold up your end and decrypt the data then no one will bother to pay you in the future. The entire "market" is best served by playing "fair", in so far as that can be applied here.

That doesn't mean some criminals won't just take the money, but it does mean that most of them wont and that the larger players have a vested interest in keeping that behavior to a minimum.

What reason so they have to not send the keys (apart from not having them)? If they send the keys, all the sooner the can hit them again.