Being able to foresee something and being able to avoid it are different things. This is the time travel paradox problem in different costume, and all the same answers may apply. Maybe the future that is seen is the only future possible. Maybe it isn't but the universe has some inertia and it's incredibly hard to keep what was seen from happening. Maybe that future happens _because_ you attempted to avoid it based on the prophecy. Maybe the prophet can't get every possible detail right and misreads the prediction. Maybe saving this person would cause worse problems later.

See also Dune's description of how hard it can be to find a path to a desired outcome through an average sea of alternatives. See also Minority Report's observation that even with multiple seers they can get it wrong.