Mostly because they would need Apple source code and toolchain, too, and dozens of coders willing to make evil updates. Of course they could get those, too. But imagine what an expensive botch some "security contractor" would make of the project of crafting evil updates for the FBI. They would probably be found out and publicly humiliated, and there would be a terrible trust hit to the whole tech industry.
So, they always bounce back from it, it's not as if they haven't botched things before.
Why don't they also design the iPhone 7 when they are already at it?