How come cockroaches ignore zombies? aren't they carrion creatures?

[Valase]

The reason they ignore each other was - as far as I remember from one pull request - that there was too much fighting going on between all the animals and Zombies in a city, so they were made to ignore each other…

Edit:
Well, small correction… I found the pull request and spicyshadow was correct:

@Pull request 15372

Zombies don’t want to attack arthropods because those are too small and won’t zombify.
[…]
Roaches don’t attack zombies because even though they probably could tank that blob poison, they have a lot of better food around.

Also, take a look at this topic, some of the discussion is similar to here:

Cockroach is zombie faction? Intended?

What faction is huge cockroach? It is a mutated animal, so I can understand if it in fact zombie faction, but there is no indication that it is, so I thought it was just ‘mutated animal faction’ huge cockroach and zombies go out of their way to ignore each other, even when I went so far as to block a door with the roach, but the zombie just pushed it into the room with me so they could both attack me.