There are lots of lenses with internal dust, which is not necessarily an issue. I think some people will point out a bit of dust contamination & scream fungus, especially if they have an interest in knocking the price down. A few really do have fungus, those that do have it do need attention if they're ever to be useful in the future.
Many lenses have gaps in the focusing or zoom mechanism which can suck dust particles in. Fungal spores are a very small fraction of "normal" dust. If they get in and there is food available (possibly pollen grains) plus dampness, the fungus can grow. The issue here is that growing fungus can eat into the coatings on the lens, so just dismantling and cleaning isn't going to fix the issue.
If the lens has always been kept with front and rear caps on in a dry place, how would it get in there.
If you're not living on the edge, you're wasting space