That's because they are not user-replaceable. Unless you are wondering why people are saying no without having seen the insides of a new MBP. In that case, I can tell you it would be extremely difficult for Apple to add a socket and maintain the current form factor. There's not much physically different between the last generation and the iX chips, so there is no reason to believe that Apple would be able to (or want to) use a socket now if they haven't in the past. A socket adds weight, thickness and cost, and probably takes away from the cooling system since it insulates the bottom of the chip from the board. A socket is a somewhat complex mechanical device since it needs a locking mechanism and a bunch of pins to touch the contacts on the bottom of the processor. It is an additional failure point when assembling the system, and securing the CPU in the socket likely can't be done on the same equipment (pick-and-place machines) that places all the other components (including the CPU for a non-socketed CPU) on the board.
I don't even know if Intel sells mobile ix chips in a form-factor that can be socketed.