Ummm, because if you need your hack to be reliable, you can't rely on someone else's bugs to be there when you need them. You never know when they'll be fixed, or just replaced by new bugs.
A long time ago when setting up computers and networks was driver version hell, we had a short list of manufacturers' computers that we'd do setup included in the price instead of on-the-clock. This came about when a shipment of about 20 Dell computers, all supposedly of the exact same model# and revision, required about about 11 different setups, because the various chips on the board were different. They were clearly just using the chip-of-the-week>from whatever supplier was cheapest -- great for their price points, but every variant required a different driver for some subsystem. So the list was created and Dell was not on it (it was IBM, Compaq, HP, DEC, to show when this was).
That's solved now by hiding it with the much more automated OS and networking setups, but it is easy to see how the Chinese spies would be in the same situation -- some buggy boards are wonderfully exploitable, but how do you tell that the version going to your target wasn't changed by some revision that wasn't even noted in the Rev- listings? Better to insert your own bug if you want to actually get the job done.