Why don't Polar Bears eat Penguins?

My creepy Birds Eye bear thing finally arrived. Which gives me the opportunity to make a weak visual gag by dusting off an old joke and giving it a bit of a polish.
So I suggest that the answer to my question could be: Because they can't get the wrappers off.
Or the more zoologically accurate: Because they inhabit different geographic hemispheres.
But my current favourite is because they can't afjord them. (It's been a couple of days and this still cracks me up)