I understand the difference between both schools of thought but why is rule utilitarianism thought to be better?

With context to this question. What is the difference between Rule Utilitarianism and Act Utilitarianism?

A key question in ethics that came to the front in the 20th century is the distinction between metaethics and normative ethics. It's not a perfect distinction and tends to blur (constantly), but the basic idea seems sound:

Metaethics looks at the principles behind deciding whether something is right or wrong.

Normative ethics looks at how ethics can guide conduct.

Some people claim, for instance, that Kant is only providing us with a metaethical framework.

The reason I raise this distinction is that it's not clear what Bentham and Mill are giving us in their formulations of utilitarianism. Are they trying to give us guides to particular conduct or are they giving us the system that tells us whether something is right or wrong?

Act utilitarianism takes the position that utilitarianism is meant to be normative ethics. In other words, I decided what is right and wrong by calculating maximum utility and pursuing that action.

Rule utilitarianism makes utilitarianism a metaethical standpoint. Here, we've pre-calculated courses of action and then work from these results.

(Again, note I'm not saying the distinction is perfect so don't nitpick it). The (supposed or apparent) disadvantage of act utilitarianism is that calculating utility could be too seen as too intensive in the moment. Moreover, the calculation should take in account epistemic gaps and other concerns that make it massively difficult.

Maybe to illustrate, let's say your son Albert has a dog named Buddy that you accidentally ran over with your car. Your son asks "what happened to my dog?" If you're an act utilitarian, it seems you should figure out in the moment before answering whether or not you should tell me the truth and if so how, while considering how it will impact his happiness, your happiness, his suffering, your suffering, his eventual life and other such considerations, all while recognizing his personality and other features.

This seems pretty unrealistic to do for every single action you need to do.

Rule utilitarianism comes to the rescue by saying we can calculate in courses of action already. No need to calculate on the spot whether lying to someone is a good idea or whether killing a drifter is acceptable (we can pre-reason to the answers and this will tell us moral courses of action).

Act utilitarianism seems to require de novo analysis of each situation whereas rule utilitarianism lets you work from "precedent".

I think the usual response from committed act utilitarians is to suggest the calculations aren't so onerous and that rule utilitarians are overcomplicating them. (the same defense can apply to a second objection which depends on an ambiguity between actual and expected consequences).

References

R. M. Hare (https://en.wikipedia.org/wiki/Two-level_utilitarianism)