Apple thinks that loud fans are a bad user experience, and I can’t fault them on that. So they design around convection as much as possible: hot air rises and leaves the case, cool air enters from below and the case geometry and plastics guide it to the areas that need it most, just by their static shape. That carefully crafted path for the air to follow is designed around the computer being right side up and it doesn’t work the same if the thing is inverted.