The reason? When sitting behind my desk at home or office I don't like to be limited with the mobile CPU. I have the required kWhs to power a proper CPU, GPU and run 64GB of memory. I also don't want to run separate computers on each location, because keeping these in sync (OS settings, applications, databases etc) is painful.
Technically we are almost there. We can put reasonably fast flash storage to the phone. USB-C should provide enough bandwidth. On OS software side we would need some work to make plugging in/out convenient. I don't want to do a full reboot every time I "unplug" the phone from desktop processing unit and move it somewhere else. As I move between processing units I would like to keep my apps open, maybe just doing a hibernate/sleep and then waking the system up connected to a different processing unit.
This solution means double spending on CPUs and memory, but desktop hardware is relatively cheap.