Because processors are now so many orders of magnitude faster than main memory a huge percentage of their transistors and complexity are allocated to things like caching, prefetch, and branch prediction.

Those features that are the only things saving modern processors from spending nearly all of their time doing nothing useful while they're waiting for data to be retrieved from main memory.

Look at a Core i7 CPU die, with the L1/L2/L3 cache circled. Understand that many of the remaining transistors are doing things like branch prediction, managing cache coherency, and prefetch and so forth.

  http://i.stack.imgur.com/4Z1nU.png

Now imagine that all of those resources could be doing actual processing instead of simply caching gobs of data that already resides in main memory.

That would be awesome.