When non utilitarian rationalists consider big life changes, it seems to me that they don't do it based on how happy that will make them, Why?

Utilitarians could say they are trying to maximize the World's something.

But non utiltarians, like I used to be, and like most here still are, are just... doing it like everyone else does it! "Oh, that seems like a cool change, I'll do it! yay!" then two weeks later that particular thing has none of the coolness effect it had before, but they are stuck with the decision for years....... (in case of decisions like job, partner, quitting, smoking, big travels, big decisions, not ice cream flavour stuff)

So, why don't rationalists use data driven happiness research, and reasoning in the happiness spectrum, to decide their stuff?