In the first 2 seasons they were looking for a cure and Liz wanted to be human again. She might miss solving cases with the aid of the victim's brain, and as it's become addictive (the taste of human flesh), it's a difficult choice: stay a zombie or be a normal human.

Now, it's becoming more like a prequel to "The Walking Dead". Everyone is being turned into a zombie, and they like it: the superstrength, fast healing and (almost) immortality. As long as they eat (human) brains they can function normally.

Soon there will be farms where humans are kept like animals. There'll be supermarkets selling brains and other choice pieces of human meat. You'll be able to get brain burgers and pizza with special toppings (pepperoni made from guess what). Why not try ice cream with amygdala flavour?