Because they are the media that reaches millions and easiest way to promote the game. Bungie rakes in millions because streamers brainwash everyone by telling them how great and wonderful everything is in destiny. Bungie sends streamer free stuff, trips, access to events, neat stuff, notariety etc. Streamer goes back to stream about all the cool stuff, giveaways, trips and worshiping bungie. We continue playing and new players get sucked in. Rinse and repeat pretty much.