Tom,
If the uncompressed data is transfered over the PCI bus, I think you are right. If the data remains compressed and is sent to a PCI video card for processing, then I think there is enough bus bandwidth. If the data is decompressed on the CPU and sent to an AGP Video card - there should be enough AGP 4x bandwidth (?).
Ideally, the tuner card would talk directly to an on-card graphics chip and the data stream would be, at least partially, processed on card and there would be no need to transfer uncompressed data streams over the PCI bus.
I'm not sure why it has not been done. A lot of people have different potential explanations. I was hoping someone "in the know" would have the answer.
Ed