I would guess because the Intel® Pentium® 4's are old, discontinued and that they don't match up well by the standards of performance today. The biggest reason is that there are lots of them out there. When supply is much greater then demand price will drop to the point where it reaches the balance between price and demand.
I think intel enthusiast hit the nail on the head. It also seems that the CPUs far out live the mother boards, so as all of those 7 and 8 and 9 year old board die due to old age, or power surge, or capcitor leakage, the CPUs are put on auction sites. I have like 20-30 p4s that came from dead boards, and it is hard to find a new socket 478 board. Even the 775 boards are getting harder to find. On top of that new CPUs and bare bones kits are so cheap now adays the value is not there to rebuild an old p4. I personally have an old p4 as a medai pc and it works great. But when it dies im going to get a netop, rather than fix it.