When not plugged into a docking station, a laptop has a slightly smaller screen, a keyboard without some of the duplicate keys, and a pointing device that works a bit differently. That is, at its most different, it's not that different. Really, the main difference is that the hotplug system gets a major workout. A phone-class device spends most of its time without any keyboard at all, with a much smaller (in inches) screen than any laptop since the 80s, and with a touchscreen (which is much better than a regular pointing device for some things and much worse for others). That's way more different from a desktop than a laptop is. And a phone-class device (in its common configuration) has a different ordering of what operations are more convenient than other operations.
Of course, it's entirely plausible that someone's "desktop computer" will actually be a virtual machine running on their phone. But that sort of user probably spends 75% of their time away from their desk, with their phone behaving in ways that work when you're standing on a ladder with a hammer in one hand. They're not just getting rid of the tower case next to their monitor, they're no longer going over to their desk any more, because the time they're on the go is all of their billable hours.