"Utilitarian" doesn't mean "acting according to a utility function". Further, many people's actions are really difficult to express in terms of a utility function, and in order even to try you need to do things like making it change a lot over time and depend heavily on the actions and/or character of the person who's utility function it's supposed to be.

I'm not (I think) saying that to disagree with you; if I'm understanding correctly your first sentence is intended as a sort of reductio ad absurdum of entirelyuseless's comment. But, if so, I am saying the following to disagree with you: I think it is perfectly possible to be basically utilitarian and think animal suffering matters, without finding it likely that happy rat farms and humane insecticides are an effective way to maximize utility. And so far as I know, values of the sort you need to hold that position are quite common among basically-utilitarian people and quite common among people who identify as EAs.