Which computer company builds the best workstation???
Its easy, well as easy as it is to cram tens of millions or transistors into something with the area of a dime, to make something small run into the GHz speed. Part of the reason the entire computer does not run at the processor clock speed is the distances between components. The distance between your video card to your processor is much longer than the distance between the components on the processor's die.
Cost is a big part too. It costs a lot more to make the higher speed components. On supercomputers a lot of the cost is in very high speed proprietary bus architectures to transmit data between the different parts of the computer. Here's another example of how important bus speed is. Some of the supercomputers being built now take regular processors or even old ones like Pentium Pros and link thousands of them together with lots of bandwidth.
Finally, I don't think that the entire computer system needs to run at the same speed as the processor clock. Lots of computing tasks require the processor to grind away on calculations for a number of clock cycles and then feed the results back to the rest of the computer. Getting the system clock to run at the processor clock would probably be overkill. But, there can definitely be improvements over where we are now.
As others have said in this thread, do your best to get a balanced computer system. Right now it seems the AMD based computers are more balanced but the Intel high end processors have more potential that's not yet available.
Apologies for rambling.