I'm inclined to believe many are empty investment properties. A documentary about the housing costs showed a Chinese investment tour of Cupertino for such purpose:

https://youtu.be/SBjXUBMkkE8?t=2m35s

The other issue is that the high school I went to there, Monta Vista High School, is extremely well-ranked. As such some investors buy houses, then rent them to a single custodian who boards as many students as possible so they can attend the schools. These custodians don't seem to have a high priority on keeping the house in great shape.