Power losses in wiring are an issue. Many of your DC devices also happen to be low-voltage. (For example, USB devices.)

If you transmit the same number of watts at a higher voltage (lower current) rather than at a lower voltage (higher current) through a wire, you will lose less power.

So for moving the power around the house at distances more than around 10 feet (~3 meters), it's more energy-efficient to convert it near the point of use. And if you're already going to convert it, it's probably not worth the expense and complication of running two separate wiring systems.