Not quite, doesn't matter how many gifted employees you have in-house you'll never catch everything. A familiar work environment breeds a familiar way of thinking, sometimes you need someone to take a look at code from a completely different perspective and it's amazing what can be spotted hiding in plain sight. Happens in all walks of life.
Increasing the number of eyes on their systems with a financial incentive is a really efficient and effective way of catching security flaws.
It has to do with what you can actually test/devellop in house vs what the whole world can test/develop. That's why you have public/dev betas and big companies buy statups and don't expect internal R&D to uncover all future tech the company will use.