Casinos should be bug-free and hacker-proof for them to gain the trust of the gambling community, there are casinos with their own created script because many casinos now are on the Whitelabel run platform, it's more cost-effective, easy to manage, and the burden is on the Whitelabel company they are the one who handles everything, that is why we seldom read of casino getting hacked it's not because hackers can't hack their system but they have experts developers to look for a patch 24/7 because that's their business, protecting their platform.
There's no system that's 100% system proof though. You can't consider a system being managed 24/7 as foolproof since it's that 24/7 management that makes it foolproof. I do agree with the Whitelabel solutions though, as I've said in my earlier post they're pretty much the same so the amount of bugs is rather minimal.
Bounties were not that popular now, it might be a personal hiring now in telegram depending on your profile and application. Since it is hard to filter now participants in bounties some look for it in other platform so they can scan applicants properly. There is not much new gambling sites to test and when there is, this forum gives review and feedbacks already about the casino same with other old casinos that have thread here, many are open now to their review.
Rather than unpopular, bug bounty is just something more specialized to people who actually know how to attack systems instead. And another factor is as others and I have said, casinos use Whitelabel solutions so the bugs that could be found would already be rather minimal.