CPUs are designed in a pipelined manner -- once the high-level team finishes up their work and passes it on to the low-level guys, they immediately start working on the next version, with the first version still years away from release. The total cycle from drawing board to store shelves is >5 years, and the skill sets required of high-level designers are very different than those required to turn it into silicon, so this just allows them all to have something to work on.

The long lag time is why chip companies can have disasters like P4 or Bulldozer, and they last so long. When some of the basic assumptions are wrong, the designers won't actually find out until after the design hits silicon, at which point the next two versions are already pretty much locked.