A thread for sharing theories and headcanons, since I couldn't find a pre-existing one. I'll start:
zombies want to eat brains because they're missing most of their own brains from the zombification process. The brains they eat goes to their skull, increasing their brainpower. Animal brains can be used for this process as well, although they're not nearly as effective as human brains.
Zombies do not instinctually know that eating brains will increase their brainpower.
Having brains is a status symbol amongst zombies, similar to how many parts of the world regard the elderly as being wise and experienced. Dr. Zomboss is highly respected among zombies due to how many brains he has eaten. Zombies follow him because they wish to share in his prosperity.
While zombies have always been present (as shown in PvZ2), the original PvZ takes place during the rise of Dr. Zomboss, who was able to organize zombies in ways to maximize their chances of getting brains. In exchange, he gets a share of any brains his followers acquire.
The sun emits light and heat in concentrated sun orbs rather than through radiation. The sun itself is much less bright in comparison to our own sun, but the light from the sun orbs offsets this.
In Soviet Russia, zombie Lenin attempted to organize zombies as well, only he mandated for brains to be equally distributed amongst the zombie population. However, this ultimately failed due to zombies lacking the math skills to evenly distribute brains among themselves. (sorry, couldn't resist)
Discuss my headcanons, or share your own. or discuss the headcanons other people have come up with!