Humans also don't have the instinctive nature to eat brains. I'd say that if zombies cannot survive being frozen, then eventually the zombie-parasite/virus will evolve, and the zombie will have migration instincts to survive winters if it doesn't already.
Humans do, however, have the instinctive nature to reproduce, to eat, and to fuck each other over, and that's what zombies are doing.
Zombies don't specifically eat brains. That's a misconception perpetrated by Return of the Living Dead, which was NOT made by a respectable film maker. In the Return of the Living Dead mythos, zombies cannot be killed, ever.