I co-manage https://hackerone.com/googleplay and the top contributor there probably makes 5x - 10x of an average software engineering salary for his home country.
Not a lot of hackers care about Android app security so there's barely any hackers participating and little competition. Most apps have never had anybody do a security review.
Additionally the scope of the program is so wide that you can look through hundreds of apps from companies that have no security posture at all. Finding bugs is easy and payouts are more than generous.