Why is it so hard to throw a bullseye in a game of darts? You know the weight of the dart and the location of the target. Why don't we hit it every time?

Even if interception is conceptually simple, the speeds and distances involved make it a very challenging problem.

Additionally, you're starting from a false premise that ballistic missiles don't maneuver. Some of them, particularly MIRV ICBMs, are highly maneuverable.