I agree with almost everything been said in OP but I will answer what I probably could here.
I just do not understand why people would want to work for free without any guarantees at all. If you are really unlucky you could spend 1000s of hours doing work for these bounties and profit nothing. I am interested in hearing about your experience with these bounties and how much you profit when it comes to withdrawing the tokens.
Because there are fewer projects and 100 times more participants. That is also the reason why you will see even scam projects or shady ones getting enough attention from bounty hunters. There is a common tendency among bounty hunters, that is - If even I don't get anything from these tokens, why shouldn't I try it at least, because it costs me nothing.