Phones are small and easy to carry around but mobility has it's weaknesses.
For example, cell phones are so heavily underpowered in comparison to absolutely anything from a desktop computer down to a little 5 inch netbook. They lack a large variety of programs that I can do on a computer including things such as C++ VPython Photoshop and any game that is of any real quality. Try running those on any of your phones-- guess what, they can't. The reason being is that the fastest phone in the world, the Droid, which has a 550 MHZ Processor is nothing in comparison to even a laptop that's several years old. There were processors in computers far more powerful than that way back in 99 with Intel's Pentium 3 series now with the current generation of Core2 Duos and beyond the average computer has as much as 6GHZ easily. As such, computers are far superior to phones due to superior power and very few limitations in programs, especially in comparison to a phone.