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.)
The only things I can think of are you really want to keep all the data on your phone and don't want to use cloud sync solutions (Dropbox etc.), or you really want to save a couple of hundred dollars getting a (probably terrible) laptop without a motherboard. Not very compelling IMO.