I'd say all of the socket A chipsets were made before the invent of PCI-E. Manufacturers want to sell you new hardware to match new CPUs. It would not pay for them to go back and "reinvent" it because there are not a lot of CPUs left for it now, nor would there be many sales. Does AMD want motherboard companies to make new high quality socket 939 and 754 motherboards? Nope. They want to sell new chips, not let people recycle old ones. That, my friend, is why we have new sockets every year or so.
Economics rule, new hardware + new standards = new sales.
Class over, you are excused.