Why is it easy to round the hexagrams afterwards, but difficult to break them beforehand?

Generally speaking, the interpretation of divination is ambiguous, nothing is clear beforehand, and everything becomes inevitable afterwards, so of course, as you said.

Of course, if you are an expert in divination, you can actually point out the direction in advance, but the Book of Changes is actually about change. Things may change before they are established, so it is not necessarily the case.

For example, suppose someone knows that he is lucky enough to have food and clothing, then it is hard to say that this person can't do anything, so how can he have food and clothing?