There's 2 types of people that will find these kinds of exploits. Black hat hackers that do it for the money, and white hat hackers that do it for themselves/openness.
The black hat hacker would have to be paid handsomely so that he could disclose his exploit. For these types of exploits I assume they would do something like sell you a PS5 with dozens of games included for $700, and tell you that you can load many more. That means that he only needs to sell 101 hacked PS5s to make more than the reward money, and he'll probably sell thousands of them before a copycat copies him stealing his profits or Sony patches the bug, which won't stop him completely since he'll probably have a big stock of unpatched PS5s.
The white hat hacker does it for fun or curiosity, a white hat hacker is usually an advocate for open source and probably trying to run linux on the thing is the main motivation for him to keep going. After they find something they'll release it to the public, usually with piracy enabling things off by default, but since it's all open source the pirates will find a way to use it anyway.
Sony is doing the smart thing and targeting the white hacker, they're the most likely to find these exploits anyway. If they made the reward money high enough that it would disincentivize the black hat hacker from commercializing his findings, it would instead be an incentive for people to find exploits. Lets say they pay $200k per exploit, they would no longer be paying these types of bounties once a year, but every month. I'd argue that paying millions a year to protect their system is valuable, but the fact is that they can get away with much less, hence $20k is just about the perfect amount of money for a bounty like this.
tl;dr: If they paid more they'd basically be creating and funding a market of exploit finders for little gain.