More people buy them assembled (vs building) I would guess.
For those that don't it's probably because they want better parts than what the factory comes with, so it isn't necessarily cheaper to buy it assembled and then change a bunch of parts. The list of tools required is pretty small for building a lower, and people who like building things might have those tools around the garage already.
Sure it's probably easier to buy a $600 M&P ready to rock, but I can build one with better parts and more features for about the same cost, and have fun doing it. The M&P may have been assembled using a more exact process, but I'll take the better rifle assembled 99% as well
The only exception to this is someone who buys all the stock factory parts and assembles on their own, and I would guess that they value they get from learning how to do it is worth the extra cost to them.