A site that shall remain nameless under pain of excommunication around here...
google VVEL and 370Z in images and it will pop up on the first page
EDIT: Whoops -- it just links back to another thread on here. I'll shoot you a PM.
I'm guessing because of the same factors that result in other tuning limits -- gas mileage and emissions. I find it hard to believe there's no wiggle room left.
Even with ignition timing, where there is not much room for advance, a couple of extra degrees can be safely dialed in, and that nets in reasonable power gains.
I find it hard to believe the valve timing is optimized for power and any other changes result in valves hitting the pistons or power loss. If the tolerances were THAT tight, it means that with just a little valve float at high revs, you'd see mushroomed valves occurring here and there, and I haven't heard of a single failure like that, even though most people bump up the rev limit by a few hundred RPM and plenty of people beat on the car.
Moreover, even if timing and overlap is optimized when stock, slap on a free flowing exhaust, different IM or set of headers and its (potentially) a different ball game.