Also because the course cannot be exactly computed in advance. The environment in space, even far away from earth, isn't an ideal vacuum and particle density will depend on solar activity. Then there are effects which are (or until recently were) ill understood, like the pioneer and voyager anomalies. I suspect however, that the effect of not quite perfect burn schedule and burn intensity of the propellant has a much greater effect. And how well do we know the mass of e.g. Jupiter really? GIGO.

> I suspect however, that the effect of not quite perfect burn schedule and burn intensity of the propellant has a much greater effect.

This is the real answer right here. Rockets have all sorts of uncertainties in them. You have the measurement uncertainty in exact orientation and the measurement uncertainty in the acceleration and thus total thrust delivered, all on top of the physical uncertainty in exactly how powerfully your engine is going to burn, and for how long. Remember, there are physical valves that need to open and close to control propellant flow, and there are chaotic perturbations in the conditions inside the combustion chamber. You simply cannot remotely achieve a perfect delta-v in a perfectly specified direction; there are uncertainties on both.

You are right, one can imagine a chaotic system where any deviation from a path will be amplified and have significant costs in the future (i.e. space navigation in realistic gravitational fields), then extreme precision may make sense (where an error might be amplified millions of times further in the path).