It's in part because old displays had a screen resolution 640x480 or 800x600 back in the 90's, only a little more than phone displays (375x667 for something like an iphone SE, but at higher pixel densities).
Since the 90's, desktops went to 1024, 1920 / HD is mainstream now, and 3840 / 4K is on the rise for desktop.
Since the 90's, desktops went to 1024, 1920 / HD is mainstream now, and 3840 / 4K is on the rise for desktop.