gddr5 not overclocked in most modern graphics cards is as fast as some of the fastest overclocked ddr3 but i am unsure about the CAS on the GDDR5 but with that said they work in different ways ddr3 and GDDR3 worked the same but GDDR3 works on speed while GDDR5 works the same on a lower speed because of the higher bandwidth so a better question will be why don't we improve the bandwidth of the RAM standards?
There are physical limits now on how thin the wafers are and how little space between the wafers can be. This is why there is so much interest in quantum computing and other tech like phase change memory (see March 2012 issue of maximum PC magazine). This is also why the Ghz wars turned into the mulitcore wars, more performance at the chip level is becoming physically limited. We have to look to new technologies to continue Moore's Law.