Any personal computer built in the last five years can do anything a smartphone or tablet can in terms of processing power and connectivity. A smartphone is a computer with hardware that enables it to use cell phone networks and make calls.

My inner cynic strongly suspects that Facebook and other similar corporations really like the control they have on the overall user experience on the two major mobile operating systems; i.e., eyeballs on a smartphone or tablet are worth more than those on a general purpose computing device.

Too much freedom on a personal computer; with browsers that feature all kind of privacy enhancing add-ons such as ad-blockers and tracker-blockers. Much harder to monetize.