Preservatives aren't to prevent rot, but instead, to prevent insect, mould, fungus, etc from eating/rotting the wood.

Moisture can come from the air, and damaged/poor air separation. Insects are, well, they're insects.

And of course, there are mice, ground squirrels, and other critters which chew on wood relentlessly. If I didn't kill 100+ mice a year, and a few ground squirrels, I'd be up to my elbows in critters, not to mention, dead from the hantavirus, and my house burned down from wire damage.