Linus Tech Tips recently published a video where they did all kinds of cooling hacks to a Macbook Air, including milling out the CPU heat sink, adding thermal pads to dissipate heat into the chassis (instead of insulating the chassis from the heat), and using a water block to cool the chassis with ice water.

They got pretty dramatic results from the first few options, but it topped out at the thermal pads and nothing else made any difference at all. Their conclusion was that the way the system was built, there was an upper limit on the power the system could consistently provide to the CPU, and no amount of cooling would make any difference after that point.

The obvious conclusion for me was that Apple made decisions based on battery life and worked backwards from there, choosing a chip that fell within the desired range, designing a cooling system that was good enough for that ballpark, and providing just enough power to the CPU/GPU package to hit the upper end of the range.