A watch is only to tell time. A phone is only to talk. Neither of these require a size bigger than "barely able to see easily. However smartphones have incorporated 'web browsing' (and analogues) into them, and they do have a size requirement that they can't get smaller than. The market has spoken, and all the vendors are bringing out larger phones with bigger screens. That functionality isn't something that can fit onto a wrist; not until we have sci-fi style holographic projectors, anyway.
Weren't watches replaced by watches?
They started off in towers, then to rooms, then to tables, then to pockets and then to wrists. Is this not a similar evolution of computers?