So these homes are vacant? From my experience in the Bay Area, these homes house their wives and small children; or their adult children. When you think about it this makes more sense than leaving them empty considering China's environment pollution.

Some of the owners move their families over. But they often own more than one property, so the unused ones are completely vacant.

I still don't understand why they don't want the rent money on those properties.

It's possible that they are so wealthy that their income from other sources far outswamps whatever rent they would accrue from the properties, so it's not worth the inconvenience of renting them out. Remember, many of these properties are purchased outright (no loan) and held as long term investments.

Because they also flip these properties on regular basis (anywhere from 3 to 24 months) and it's significantly harder to do that with people living in.

It's an inside job thing. These people basically trade within themselves to prop up the property price.