Now if there will be a real zombie outbreak, that's the time that we can take note of "factual" characteristics of zombies. They may or may not eat each other and we will never know for sure until that happens
Anyway, in a make believe world this is some speculations that i can offer XD
If a zombie is resurrected by a virus, I think they dont eat each other because they are controlled by the same virus, the same way some animals dont eat their own kind. Maybe zombies are braindead but there is definitely something controlling their lust for flesh and in this case it's the virus You may say that a virus doesnt have a brain and doesnt have the capacity to control a zombie but it's possible that the virus will only kill specific parts of our brain and leaving out the parts that has something to do with wanting to eat a human.
IIRC there are some insect that can "zombify" another creature and control them