That will be the equivalent of a 21 year old with no backgrounds robbing a bank. He'll just get caught, somehow. Either through financial transactions, taxes, leaves a trace somewhere, get involved with that criminal organization, etc... He'll eventually mess up.

But back to your question. If I remember correctly, the big companies started giving higher bounties because they noticed that some of the bugs are being sold on the dark market. Some of these bugs are certainly not unmarktable but the companies have to keep the promise.

Also I'm not an expert on these things. Maybe there is an entity buying whatever bug you can find and giving generous money. Think governmental institution who might be interested with whatever backdoor they can get.