Because Bungie keeps [intentionally] removing the incentive.
When they add high ground, they put another place on the map with a bunch of cover to camp, therefore removing any reason to contest the high ground. When they have a good mix or high ground and safe spots, they leave the spawns open. When they get the spawns and height variation right, the sightlines are cluttered. Or they'll get the layout perfect, but it'll be too flat and then they'll throw a bunch of line of sight blockers around the place to break up the sightlines. I'm not going to claim to be some map guru but these are some basic tenets of design that I learned from them.