It's not like there is one single standard DC voltage that everything runs off of. Switching between different DC voltages incurs a loss just like switching between the current AC standard and a given DC voltage incurs a loss.
If one were deploying everything from scratch, one could pick a standard. Right now, everyone is going to want to run the stuff they have, and the AC to DC converters on that stuff, even when they are exposed (i.e. wall-warts) instead of embedded in the device, are converting to a variety of different DC values.