Apple is actually just really terrible at thermal design, simple as that. It's surprising, because you'd kinda expect them to at least be slightly competent (like they are in other areas), but they're just not.

They've gotten around it with M1 by building an entirely new chip that's power efficient enough that it practically doesn't need good thermal design to perform near it's peak. It's an impressive technical accomplishment, but it's also hilarious that Apple had to go this far to cool a chip properly.