There's actual competition in retail. The mobile phone operating system duopoly isn't real competition.
What are you referring to here? If you're referring to bugs in Apple's own operating systems and APIs, they should invest in finding and fixing those regardless of whether they're paid to by third-party developers, because Apple's own apps will be impacted by them too.
If you're referring to bugs in third-party apps, Apple...doesn't really do that? The only case where they arguably do is App Review, and in my experience they'll only reject a build when a bug actually prevents them from reviewing the app, such as a crash on launch. Regardless, just like Apple should invest in finding bugs in its own software regardless, developers should thoroughly test their own code before submitting builds to the App Store.