The problem is, they generally have no direct experience of the natural world. If you watch birds, for example, you're well aware species like the northern junco or eastern meadowlark vary continuously with geographical location across the continent. And if a species can vary with location, why can't it vary with time? But if you've never looked at juncos, or meadowlarks (or many types of warblers, or the hundreds of other species that show clinal variation) you might well think species are somehow well-defined and immutable, rather than just convenient categories.
Exactly and I've observed that many creationists think that the number of species is already fixed as one can see in questions as in "why doesn't a cat evolve into a cow?".
This implies that a population can only jump from one "drawer" into an other but the extremely likely possibility that this population may evolve into something that doesn't exist yet seems to be ignored.