Why is not the northbridge circuitry a part of the cpu?

Question: why is not northbridge functionality is not a part of the
cpu?
At least for CoreDuo. The reason I'm asking is that the son of a gun is
hotter
than cpu anyway. Are there dual socket CoreDuo mobos?
I haven't seen any. Dual socket is only a Xeon thing?
I assume having northbridge for dual or a quad machine separate from
cpu would
make sense, but for CoreDuo??? It's not like there is a lot of memory
types to
support these days.

.