For the same reason computing used to be defined by a Commodore 64 more than by an IBM System/370-XA mainframe from the same year — they're the most commonly and most easily accessible computing devices.
Old farts like us think the desktop is the default kind of computer, but it isn't. Most computers are phones, followed by tablets and laptops with touchscreens, and desktops are the weirdest ones.
It's not a question of what's the most common. It ought to be a question of what capabilities do you think of when you think of "a computer". Most people do not think of their phone as "a computer", even though us tech heads all know that it is literally just that.
We need to follow the lead of most people here, and recognize that the phone is a deliberately limited device and its capabilities do not define what "a computer" is or should be or could be.
Old farts like us think the desktop is the default kind of computer, but it isn't. Most computers are phones, followed by tablets and laptops with touchscreens, and desktops are the weirdest ones.