The existing 915G & P chipsets were supposed to be for value systems.
Why didn't they make these chipsets themselves cheaper?
The i915G and i915P are definitely not "value" chipsets, they're just
not the super-expensive chipsets that the i925X(E) is. Basically
there are no OEM systems that sell using the i925X chipset (I think
Dell has one that they sell with a P4EE, marketing at people with more
money than brains).
Again, it all comes back to market segmentation. Intel likes to have
lots of different products (all made from the same silicon) that they
can toss into a variety of markets and price-points. They need to
have enough extra features and enough of a spread to justify the super
high cost at the top end while still being able to sell products down
into the low-end.