Why are PC power supplies internal?
Desktop PCs already have a chassis, and it already has to deal with containing emissions.  The unit as a whole will already need emissions testing and verification.  There is really nothing to gain from a external power supply.

On the flip side, now the power supply itself can be made more cheaply since it doesn't need to have a finished case that keeps the user from the high voltage, doesn't radiate on its own, etc.