Because the calves are a strong muscle group and, at a certain point, trying to go heavier and heavier and heavier gets silly and unproductive.
So chasing weight, like wanting to eventually calf raise 900 pounds, isn't the best approach. Going heavy isn't the same as what I'd call "chasing weight". Going heavy - using a weight that limits you to a relatively lower rep range - can be just as beneficial for calves as any other bodypart.
Something to consider is that getting a good contraction in the calves is generally key to growth, and that can get tricky when you start getting into super-heavy weights (again, just like many other bodyparts).
One method I like to use is supersetting flat-foot (on the ground, not the step, so using a limited ROM) heavy raises with full ROM/deep stretch lighter raises.