Actually they invent them for money. Things that exploit the OS can sell for hundreds of thousands of dollars on the black market, or hackers can just use them themselves like a business. If not for money then for recognition so they can sell their next one for money, fortunately for macs nobody wants to pay hundreds of thousands of dollars for an exploit that targets 5% of computer users. OSX has already been brought down, more than once, but they were at hacking contests where winners get a bunch of money but they have to tell how they did it and how to fix the hole, it was pretty much proven that OSX is the most insecure OS.
Yeah but at those contests the hackers had direct access to the machines, didn't they?