I'm sure Apple has done some pretty exhaustive (no pun intended) thermodynamic analysis of the case design.
I have three macbooks, two airs and a 13" mpbr, and none have had any thermal issues at all, even in the heat of summer.
If you are using a laptop in a way that generates that much heat, you probably get about 1 hour of battery life, which sort of defeats the purpose of having a laptop in the first place. A larger fan or heavier case material would also make the laptop less user-friendly for the vast majority of users.
I have three macbooks, two airs and a 13" mpbr, and none have had any thermal issues at all, even in the heat of summer.
If you are using a laptop in a way that generates that much heat, you probably get about 1 hour of battery life, which sort of defeats the purpose of having a laptop in the first place. A larger fan or heavier case material would also make the laptop less user-friendly for the vast majority of users.