These are too broad
Without specifying a particular work, questions like Why don't zombies eat each other? are far too broad. Virtually any answer one wants can be correct, depending on whether one can find a work containing a "zombie" that follows the right principles.
Is it because:
- The virus in them seeks to infect human beings?
- They are reanimated corpses that simply obey the whims of their animator, and thus have no desire to attack their fellows?
- They are humans placed in a near-death state, and those don't want to eat humans' brains anyway?
- They thrive on life energy, and the undead don't possess it?
- They are evil, and see no reason to attack other evil creatures?
- They are actually aliens, and the "brain-eating" was a myth made up by frightened humans?
Any and all of these could be true, and most of them likely are nothing close to the sort of answer the OP wants.
The accepted answer on this question ends thus:
In short, it's a conceit of the genre, and each author gives it his
own spin (or doesn't - some just don't address it).
Such questions are the definition of "too broad." They can have any of a very large set of answers, and producing a complete answer is basically a matter of creating a large set of mutually contradictory answers and combining them into one.
Fortunately, I believe most such questions can be made clearly on-topic by clarifying what kind of creature or situation the questioner has in mind, and asking about a specific work that exemplifies that situation. Thus the question becomes narrow, with well-defined answers.
For example, perhaps the questioner has Romero-style zombies in mind:
Question: Why do zombies in the Romero films not attack each other?
Answer: "...they seek living flesh. If it isn't warm, they leave it be."
Similarly, Should werewolves in literature be considered magical living creatures or part of the Undead? has two equally correct, entirely contradictory answers: yes and no. But specify a work of fiction (and the questioner probably had one vaguely in mind in any case), and the question becomes much more clear:
Question: In the Parasol Protectorate, are werewolves undead or alive?
Answer: Werewolves are considered a form of undead.
Question: In the Dungeons and Dragons, are werewolves undead or alive?
Answer: Werewolves are living shapeshifters. Although they are vulnerable to silver, they are not undead.
Indeed, there very premise of such questions is often incorrect if applied to every instance of a category.
Why are vampires not rotten like other undead?
The answer varies from "magic" to "no, they are most certainly rotten." Specify a universe or narrow set of universes, and the question becomes better-posed.
With all such questions, we should pick a work that exemplifies what kind of creature the questioner is thinking of, and run with it.
That way we get to keep the question, but actually give it a proper answer, rather than a (possibly unbounded) set of answers. In addition, there's nothing preventing one from adding contextual genre information (if such a concept is well-defined) to such an answer.