Have you ever used a Smart Phone to attempt to do any of the things you do on a Desktop?
I would guess not as they are underpowered, Under resourced and generally speaking way too slow for anything but what they do and even that is slow compared to what a computer does. Sure they are portable but being considerably slower and far more expensive to use is the killer here.

OH and would you be willing to leave your Phone at home so that it could be used by anyone who wanted access to the computer? Isn't it better to have a Single Function Device available for those who want to use it and your device available for you?
Col