If the house is also an investment, they can make money by buying rather than renting it. This is part of the problem with so much speculation and housing being treated as an investment: you earn potentially greater returns by buying a house, so everyone starts doing it and th values keep going up.

As to why they don't rent it out when they aren't in town, I can think of a few reasons: they don't want the risk of items in the house being damaged, they don't want "other people" in their house, they like visiting on short notice (even if it's only a few times a year) which isn't compatible with renting.