Lack of use kills Zamas. Walbro fair better in my experience. Not sure about Tillotsons.
I think it's just general lack of care. I am guilty myself. I never changed the foam air filter element in my Mitsubishi TU26 engine on my weed whacker for it's first seven or so years (albeit used infrequently) and when I finally decided it was time to check how dirty it was, it fell apart in my fingers. Lucky a piece wasn't sucked in during use.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!