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?

"Non-utilitarian" doesn't equate to "ethical egoist". I'm not a utilitarian, but I still think my big life decisions are subject to ethical constraints beyond what will make me happy. It's just that the constraint isn't always (or even usually) the maximization of some aggregate utility function.