[F]ormer and current Apple employees said there's a "massive backlog" of bugs that have yet to be addressed.
It's pretty obvious from using Apple software that this problem extends beyond security bugs. Submitting bugs or feedback to Apple is seldom fruitful and submissions almost always go unacknowledged. Many reproducible bugs go unaddressed even across multiple major releases of macOS.
I don't work for Apple and I don't pretend to know what the problem is, but it's becoming more and more apparent that Apple's current engineering and QA practices are not working well for them or for their customers.