I agree with you that joining projects that are already listed is much better it is a common sense thing but what you are saying is close to none exist because there are no listed projects that will pay 75k-100k for bounties, you can find listed project bounties but you will find them paying pennies which if you compared with the amount of money the risky projects offer then taking the risk will be much appealing.
 
For example, taking the risk to do bounties for 5 projects that offering to pay 1000 each is much more appealing than accepting to do bounty for a project that offering to pay 10.