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.