[CyberFonic]

The short answer is that clock rates are plateauing. Once you pass 2GHz heat, electrical characteristics, etc start becoming very big issues. As far as I can recall there are hardly any CPUs around that go much past 4GHz.

So if you want to increase throughput you start running multiple CPUs in parallel to increase the throughput. If you were to have discrete CPUs then the wiring between them, memory and I/O becomes another serious issue, so they try to integrate as many CPUs onto a single die in order to reduce the electrical delays, etc.

People often forget that in 1 ns (i.e. at 1GHz clock rate) light travels about 1 foot and less than that in electrical conductors. So keeping conductor lengths is a very big issue.