Besides cost, two other reasons come to mind for not making rerouting too sensitive: (1) I don't know if the positioning accuracy is always guaranteed to be correct and in sync -- e.g. what if when entering a tunnel your positioning throws quickly while the accuracy lags? And, (2) there are normal small deviations, such as lane choice on big motorways or intersections, or detours around road works, which can be maybe up to 50 meters.