Zombies as enemies because their AI is easy to code?

Isn't that what it basically is? Charge and rush mindlessly without fear of personal safety.

One doesn't even have to provide a coherent back story for them because ... zombies. Reminds me of the progression of enemies in Dragonball Z from the Cell Saga to the Buu Saga. Why bother writing intelligent enemies when you could just have a mumbling babbling idiot be all powerful.

Have you ever coded Zombie enemy AI?

It can be as simple or as complex as you want.

Nowadays off-the-shelf military enemy AI is incredibly cheap and easy to implement, so are a lot of other kinds of AI. So choosing a type of enemy for your game, especially if you do have a large budget, isn't really anything to do with how easy it's going to be to code.

Plus with Zombies you have to make certain art and rigging considerations if you want them to have things like limbs that fall off, or legs you can blast off and then they switch to a set of crawling or limping animations.