They change but much more slowly since they change with long term changes in terrain, as opposed to the natural weathering effect of a river, especially one that meanders.

Plus greater watershed boundaries tend to settle around mountain fronts or peaks as well as other such difficult terrains that are less prone to hosting large settlements that could hypothetically get cut off from their own greater metro area, and more likely to already form their own natural barriers just because that happens to be the places where water will begin moving in the other direction towards a different outlet.