Because Apple doesn't have a software testing culture. At least that's what I've been told by Apple engineers I know who used to work at Microsoft ("software testing is not the same respected discipline you're used to at Microsoft"), as well as when I interviewed with Apple and asked what kind of test team would be backing the product I would be working on (manual testing with no SDETs, apparently).

Take it as the quite anecdotal evidence it is, but to me it explains a lot.