A simple way to think about it: longer lenses magnify more, so blur is magnified more too. That makes depth of field seem shallower.

Another key point is that f-number is a ratio, not the physical hole size. At the same f-stop, a longer focal length has a larger actual aperture opening, which also reduces depth of field.

But focal length alone is not the whole story. If you keep the subject distance the same, a telephoto lens will usually show less depth of field than a wide lens. However, if you change your distance so both lenses give the same framing/angle of view, the depth of field becomes very similar, with only small differences.

So in practical terms:

• same distance + longer focal length = shallower depth of field
• same framing (by moving closer/farther) = depth of field is nearly the same

This is why the “wide lenses have more depth of field” idea is often true in real shooting, but mainly because of magnification and shooting distance, not just focal length by itself.