There're probably many factors involved that make it difficult from a cost-effectiveness POV to implement "extreme cooling".
I mean, you have to make it available to many mobo layouts with the pci so you don't alienate a large base of consumers. Niche products that only really appeal to hard core gamers/ethusiasts. It would be too expensive for this group of people to prop up and it would die off eventually.
Look at the ABIT OTES system. It's been held to high regard by many reviewers - but issues like taking up dual PCI slots due to it's size and the noise that it makes cause many to think twice. Because then you run into other issues like people wanting their pci slots or people wanting quiet systems and not willing to take on an OTES system.
So, then you pretty much leave the hardcore gamer who doesn't care about comp system noise and is willing to spend bigger bucks than normal for such a niche product. That doesn't leave many people to justify the expense and the very real probability of failure/non-acceptance.
Anyways, that's my thought on the issue.