Late reply, I know :( - Actually, it's not even the cooling outside the CPU that's the real problem, it's moving heat away at a microscopic level near each transistor. With enough power going through enough circuits nearby, the material the CPU is made out of becomes the issue.

Consider a block of metal submerged in a (hypothetical) liquid at near 0 degrees K. Despite how much heat this liquid can draw out of the block, if heat is being generated too quickly at the very center of the block, it's possible that the heat conductivity of the metal block itself limits the efficacy of outside cooling, resulting in an "overheating" center.