After completing the Zen design. From leaked benchmarks it looks like he did an excellent job on the design (yet again) and they have been testing and ramping up for fabrication since he left. After they have milked this design with iterative improvements for 5-10 years maybe he will come back for the next generation.
He did not just complete Zen, he completed the high-level design of Zen and it's two successors.
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.