Stupid, or seemingly so, question

So we're all getting pysched with Winbond launching their all new BH-4 4ns chips. Great...but some Geforce3 Ti's had 4ns RAM ages ago. Now we've got video cards with lower than 3ns, and the speeds to match. With the latest motherboards clearly being able to support speeds beyond DDR500, why don't we see super-low rated memory chips being used? I'm sure this has been asked before, but I can't find any answers, nor figure it out. Sure they produce a little more heat and draw more power, but video card PCB's can handle it fine, why can't memory modules' PCB's? I heard that video memory also uses higher latency, but don't see why this is an issue either.

look at a video card, see the heatsink/fan on the chip? Now for memory, all those pcbs get hot. How about we get a heat sink and fan on every little pcb we have. w00t!

Super low memory is a lot more expensive. Just like an athlon XP 3000 is $250 and a 3200 is $450, a little costs a lot more.
These chips also aren't that much better, and have more error tolerance. Video ram errors result in tearing and other defects. System ram errors result in the dreaded BSOD.