The truth is that the exploit acquisition market has many legal issues. Zerodium, who is often thought to be the leading buyer, publishes misleading guides and has had unusual timing in between the initial disclosure and hacking attempts on the researcher themselves. Other buyers have non-negotiable sale (not license!) contracts that may result in your zeroday being misused, and you may find yourself in a conspiracy. And those are the reputable and responsible buyers, there are others outside the US that are fronts for Israel/UAE/China. The market has plenty of room for correction, but there's a shortage of ethical buyers.
If you could easily sell an exploit outside of a bug bounty program for more money, you'd see more people doing it regardless of the ethics (see: the NSA doing a bulk of the hiring in infosec, noone I spoke with that applied cared about the illegal surveillance disclosures and said they chose it because they offered 100k+). So the researchers currently have no choice, and the bounty programs take advantage of that. When the pendulum swings the other direction, you'll see bounty programs becoming more fair/lucrative.