Why did that stop them, and not Google? I feel like you're refusing to address the main point here - google found this bug. It happened. They told Microsoft about it, and Microsoft hasn't fixed it 100% yet.
The OP asked why Microsoft didn't find this first. The answer is too many things to test. The entire premise of the argument is insane. If microsoft had a viable way to test ALL of the their products for ANY problem, don't you think they would be doing that?