My personal theory is that the people who experience this problem are people who produce or invite an above-average amount of particulate into their work space (whether it's cigarette ash, bread crumbs, skin flakes/dandruff, cat hair or whatever) and into their laptops. Clearly this problem exists, but at the same time, the majority of owners aren't affected.

I used to work with a guy who smoked at his desk (back when indoor smoking was less of a faux pas), and his mechanical keyboards were full of ash.