Bug bounties often involve a lot of risk for submitters. Often the person reading the report doesn't know that much and misinterprets it. Often rules are unclear about what sort of reports are wanted. A pay to enter would increase that risk.
Honestly bug bounties are kind of miserable for both sides. I've worked on the recieving side of bug bounty programs. You wouldnt believe the shit that is submitted. This was before AI and it was significant work to sort through, i can only imagine what its like now. On the other hand for a submitter, you are essentially working on spec with no garuntee your work is going to be evaluated fairly. Even if it is, you are rolling the dice that your report is not a duplicate of an issue reported 10 years ago that the company just doesn't feel like fixing.
Honestly bug bounties are kind of miserable for both sides. I've worked on the recieving side of bug bounty programs. You wouldnt believe the shit that is submitted. This was before AI and it was significant work to sort through, i can only imagine what its like now. On the other hand for a submitter, you are essentially working on spec with no garuntee your work is going to be evaluated fairly. Even if it is, you are rolling the dice that your report is not a duplicate of an issue reported 10 years ago that the company just doesn't feel like fixing.