Doors are a major hole in household insulation, letting warm air enter in the summer. This applies even when the door is closed.
Furthermore, the exterior side of the door and door frame faces the elements, taking a more brutal beating from the weather than anything inside your house.
Waterproof doors need to be strong, durable and lightweight to be able to withstand heavy usage. At the same time they need to be hygienic and aesthetically pleasing.