For servers, a number of data centers I have been in do use DC power within each rack or group of racks. It removes the double conversion, and AC basically is a battery charger (this is over simplified). It simplifies the entire system removing a number of components that would otherwise be in traditional UPS systems.

IIRC, Google does this for their racks and I know they have published a number of papers on it.

For home PC's it doesn't make sense given what consumers are used to with AC power. So the current methodology makes sense, but server farms is quite different.