Comment by eru

Than what do you save? Only the system-on-a-chip (CPU, GPU, RAM).

And the hardware to get an SoC with phone-like performance in a laptop or desktop form factor is relatively cheap, just because you have so much more space and power and cooling to work with.

(Your laptop-shell definitely needs its own power supply, whether that be a battery or a cable, because the screen alone will take more power than your phone's battery can provide for any sustained period of use.)