Let's set aside the question of the reality of nerve strikes for a moment, which is a controversial issue.
The solar plexus, the floating ribs, the 'button' of the jaw, the liver, the temple, the occipital lobe- why bother going for pinpoint accuracy on a mobile, active, resisting opponent (an exercise difficult at best) when there are much larger, easier targets that take your opponent down just as efficiently as nerve strikes are said to?
EDIT: Additionally, I'd like to address a misconception that appears to be fairly widespread, as it has appeared in more than one answer subsequent to mine. Pressure point or nerve strikes are *not* illegal in any major Mixed Martial Arts promotion, including the UFC: the Unified Rules of Mixed Martial Arts, used by the UFC and many other promotions, do not include 'nerve strikes' or 'pressure point strikes' among their listed fouls, or indeed mention them in any part of the rules. The only things that might be construed as even limiting nerve or pressure point strikes is the prohibition against throat strikes and the rule against "pinching" the flesh, which would prevent the application of some few pressure points, but not most, and which is rarely enforced at any rate.