Dumb question, why do “sensitive” spots on the body need more nerves? Couldn’t you just have the normal touch-sensing nerves and map signals from specific spots on the body to stronger/pleasurable qualia in the brain?
The brain does do some of what you’re describing though. The somatosensory cortex gives disproportionate space to certain body parts (the sensory homunculus). So there is central amplification, but it works on top of peripheral density, not instead of it. Without the dense nerve input, you’d basically have an on/off switch instead of nuanced sensation.