I used to wonder about it, but I guess somebody still has an insatiable demand for computers, although I don't know who it is. There are no new "killer apps" which demand 2+ GHz microprocessors, but that doesn't seem to matter.
I know our company purchases maybe 1/3 the machines we were buying in the late 90's. There's simply no need to upgrade. We haven't found one secretary in our company who can outrun MS Word on an "ancient" 1 GHz P3.