Main reason is optics. The definition of F-stop is:
f/stop = focal length / diameter of effective aperture (entrance pupil) of the lens
The kit lens is 18-55mm f3.5-5.6. That's a 3 x zoom ie 3 x 18 = 54. There would nominally be a commensurate corresponding shift in F number of 3 stops. However lenses are typically designed to have a lesser change than the change in focal length would warrant. You can get an impression of how much you can zoom before the f-stop changes with the lens on the camera, metering off eg a white wall (an approximate measure, the camera exposure changes in steps of 1/2 or 1/3 stop depending on your camera settings).
Old film kit lenses were typically something like 28-70mm, =2.5x zoom, and f3.5-4.5 would have been pretty typical for these. I guess it was relatively straightforward to design these lenses like this, specifically because they were typically retrofocus designs with a large negative front element, so I guess the entrance pupil could also change while zooming through the wide angle focal range. There are also lenses where you can see the iris change size as you zoom.
Last edited by marcusBMG; 09-27-2022 at 11:28 AM.