- Perhaps there is a set of widespread cognitive biases that leads to a bevy of experts willing to defend an obviously untenable theory. All the expert proponents are making the same logical mistake, which can be traced back to the same psychological impediment that many of us unfortunately share.
And then there's the worry of straw-manning: how confident can you be that all utilitarians fail to realize this one mistake that you are accusing them of making? It's more reasonable to assume that sophisticated utilitarians recognize and (at least attempt to) avoid whatever mistake you think they're making. If a criticism is easy enough for a non-expert to see, it's probably easy enough for an expert to see (and deal with).
- Perhaps we can dismiss some experts because there are non-rational forces at work that explain their allegiance to a theory. Maybe there are practical reasons (books on theory x sell well; there's a lot of funding opportunities for advanced study in theory x), or cultural/institutional reasons (a society has a tradition of raising their children under theory x; a prestigious department has a long history of theory x training).
Furthermore, it's difficult to show that non-rational forces uniquely impact one theory regarding a given issue. If such non-rational forces apply equally to Kantians and virtue ethicists as they do to utilitarians, then we cannot dismiss utilitarianism in favor of these other theories. For instance, the objection, "They're getting paid to say it's more complicated than it really is! Their livelihood as a pro philosopher depends on desimplifying the obvious!" applies equally well to all ethicists.
Anyway, if someone were to offer this argument, she wouldn't be baffled as to why there were so many advocates of the crazy theory. She'd remain upset, of course, but would at least understand why this happened.
So here's an added moral: If you are baffled as to why a seemingly crazy theory is semi-popular among experts, you should look to blame yourself ("I don't know enough about it!") before you blame the experts ("They're all stupid!").