Why don't whole house fans vent to the outside?
There are three reasons why whole house fans generally vent into attics. First, whole house fans are designed to pull air from the highest point in the living space of a house, which is generally where the warmest air collects. This removed air is replaced by cool air entering windows in the first floor, flushing the warm air out of the house. The highest point is generally the ceiling of the top floor, so the obvious route for the air is at least into the attic. However, whole house fans are designed to handle large amounts of air, so it's better to vent the fan into the attic rather than directly out an enormous vent in the roof or gables.

Second, the attic is often the low-rent district of the house, which makes it a good location for the fan mechanism itself.

The third reason was raised in another answer: cooling the attic will reduce the heat load on the main house. However, a properly insulated house (at least in non-tropical climes) will have enough insulation between the top floor ceiling and the attic itself that it won't matter much what the attic temperature is; the heat won't have much of an effect on the floors below. Still, there is some benefit to having a cooler attic.

Add all this together, and you find the majority of whole-house fans will vent into the attic, with the attic in turn vented to the outside by any of a wide variety of (generally passive) methods.