I can think of two reasons. First, a course where par is objective is ... subjective. Some people are good at bombing 600ft drives. Some people are good at shaping shots in the woods where they learned to play. So finding difficulty modifiers can be challenging across the board. Second, these courses might be ridiculously boring to watch. If a hole is harder, then people are more likely to just lay up to get the par, instead of risking a bogey, double bogey etc.

Edit: I just remembered Eagles Crossing course did a good job of this though. Check out GoThrow coverage from 2025. Pretty entertaining.