Corporations innovate through the work of human minds. Corporations improving doesn't cause the human minds to improve, so there's no recursive self-improvement. Corporations today still have the same kind of human brains trying to innovate them as they did in the past.

A human+ level AI would be able to understand and improve its own hardware and software in a way we can't with our own brains. As it improves itself, it will get compounding benefits from its own improvements.