I wondered at first - but its really simple.
The focus ring has a very wide range of operation.
Quite simply if the target is as close to the lens as possible, focus will produce the largest magnification and at the other extreme the microscope will focus at infinity which is like fully zoomed out.
The concept of zoom does not exist quite like a digital camera where you cannot necessarily move the camera to the target when trying to focus on a bird close-up, but no such problem exists with the objects you are trying to view in the microscope as you can quite easily get close as you want, so no need for a complex zoom lens system. Hope this helps. Regards, Alan.