Because focal length matters a lot, not just the f-number. At a given subject distance, a long focal length gives much less depth of field than a wide focal length, even if the long end has a smaller maximum aperture.

For example, focused at the same distance, 18mm f/3.5 can keep a very deep zone in acceptable focus, while 105mm f/5.6 can reduce that zone to only a small fraction of a foot. That makes the background look much more blurred.

There is one important nuance: if you keep the subject the same size in the frame, depth of field is technically similar at the same relative aperture, but the background can still look more blurred with the longer lens because the background is magnified more.

So in practical shooting, zooming to 105mm usually gives stronger subject isolation and more visible background blur than shooting at 18mm, despite the smaller maximum aperture.