By certain level of maturity, you come to the realization that the world is way more complicated than we first predicted. Your prediction was either oversimplified or too vague. Which is not too surprising, because, if you think about it, most things have no way of being so obvious beforehand, unless we’re talking about having hands-on experience and preparation for sufficient amount of time.
Another notable point is that, areas you have considerable experience in, more often than not, cannot be explained to great extends, which, subsequently, means that there is some limit to learning in theory. To the degree the theories can be explained, at least some part of it will be some sort of interpretation, which is subjective. Not that they’re of no use, it’s just more important to have a judgement to know when things they stop being useful.