It's a standard sci-fi trope to assume that you can't just fly around a blockade, even when that blockade is just a small rectangle of ships, as opposed to a sphere.

The closest analogy I can think of is the Star Trek: Next Generation episode where there is a net of federation ships scanning for Romulans sneaking out of the neutral zone. The first question any intelligent person would ask is "the net is a relatively small two-dimensional grid, they could just fly around it."

I would hope that some script writers would actually have a reason for this standard cliche, if not actually shown in the movie, then perhaps in their heads when they're writing it.

If I were writing it, I'd probably make the weak argument that the orbital mechanics of the particular solar system prevent approaches along any other path. Or that such other approaches are logistically much more difficult because of the amount of energy that you would need to expend. This argument is especially weak in any 'verse where FTL travel allows you to pop back into realspace at any chosen point. As opposed to 'verses where FTL travel requires the entry point to be someplace specific.