Sooner or later, either you or your team will be faced with the question, ..."How come that the testers haven't found this bug?" One thing is for sure:
Testers don't create bugs, those are implemented by the developers!
So, the question should rather be:
"Why do we allow developers to implement bugs? And why do we allow them to do it over and over again?".
I cannot think of a single defect where our testers don't ask themselves the question "How could I have avoided this bug to go un-detected" and then update their testware accordingly. But the number of test cases grows with the features while the number of testers is often a constant. What's left are priorities and supernatural smell for defect-finding...sometimes it just doesn't come up to everyone's expectations.