Why is this? Well, utilitarianism says that we ought to do whatever would maximize utility. But attempting to reason in a utilitarian fashion tends to have disastrous consequences, and fails miserably to maximize utility. Therefore, we ought not to reason in a utilitarian manner. Instead, we should try to inculcate those dispositions and attitudes, and abide by those principles, that would tend to promote utility. That is, we should be honest, compassionate, loyal, trustworthy, averse to harming others, partial towards loved ones, and so forth. We should, in other words, be virtuous rather than scheming.
J.L. Mackie (p.91) offers six utilitarian reasons for opposing "the direct use of utilitarian calculation as a practical working morality":
- Shortage of time and energy will in general preclude such calculations.
- Even if time and energy are available, the relevant information commonly is not.
- An agent's judgment on particular issues is likely to be distorted by his own interests and special affections.
- Even if he were intellectually able to determine the right choice, weakness of will would be likely to impair his putting of it into effect.
- Even decisions that are right in themselves and actions based on them are liable to be misused as precedents, so that they will encourage and seem to legitimate wrong actions that are superficially similar to them.
- And, human nature being what it is, a practical working morality must not be too demanding: it is worse than useless to set standards so high that there is no real chance that actions will even approximate to them.
All of this is taken on board by R.M. Hare's "two-level" utilitarian theory. The basic idea is that there are two levels of normative ethical thought (plus the meta-ethical level).
The intuitive level is our everyday, practical, working morality. It concerns our moral dispositions, attitudes, emotions, and the general principles we tend to abide by. This is the level of common-sense morality, and it tends to look nothing like naive utilitarianism.
The critical level, by contrast, is when we critically reflect upon our intuitive-level principles. If beset by a 'moral dilemma', in which we have a clash between our principles, we will need to reason about how to resolve it. Or, in moments of cool reflection, we might ask for justifications of our intuitive-level principles. (Why be honest rather than manipulative, or partial rather than coldly impartial in our relations with others?) This is where utilitarianism comes in.
Utilitarianism is a critical-level theory, not an intuitive-level theory. The idea is that utility justifies our intuitive-level principles, if they are justified at all. (They cannot justify themselves, after all - that would be empty and circular.) It's very important to note that utilitarianism is not an intuitive-level theory. The six reasons outlined above show why it would be a poor choice. Now, it's true that our intuitive principles will be imperfect: that is, perfect adherence to them will sometimes thwart utility, and thus be "wrong" in a strict sense. But these are cases of blameless wrongdoing, as it is right for us to inculcate the dispositions that would lead us to perform those actions. For that is what will, on the whole, tend best to promote utility.
There are two upshots from this. Firstly, anyone who would directly employ utilitarian reasoning should be sternly argued with, and made to see the error of their ways, before they do serious harm. Secondly, the common purported "counterexamples" to utilitarianism are in fact no such thing. Utilitarianism praises us for our reluctance to harvest organs from an innocent passerby to save five lives. That's exactly the attitude we ought to have, according to utilitarianism! (It's only in very rare and unlikely conditions that such a repugnant action would truly be utility-maximizing, and I've previously given what I consider a quite compelling argument in support of the utilitarian conclusion in such rare cases.)
P.S. See here for my defence of utilitarianism against the "separateness of persons" objection.
Mackie, J.L., 'Rights, Utility, and Universalization' in R.G. Frey (ed.) Utility and Rights. Oxford: Basil Blackwell, 1985.