Computer:
It uses microprocessor with higher power up to the order of Watt
Cell Phone:
Low power processor means save battery, reduce heat, lower speed, and much more minimums.
Computer:
It has a north and south bridge, BIOS, super power graphic cards, and not to mention DDRs with tremendous power consuming.
Cell Phone:
none
Computer:
You can put a massive heat sink, bunch of fans, and can weight up to 25Kg for desk top or several Kg for laptop.
Cell Phone:
No one wants to buy a 25Kg phone.
Computer:
You buy it at a computer shop.
Cell Phone:
You buy it at a cell phone shop.
Computer:
To call your friend with PC, you cannot place it beside your cheek
Cell Phone:
you can do that.
and many more.