Why can't doctors lengthen the collar bones or the scapula in order to get broad shoulders?

It might be possible, but it is extremely painful and it's a slow process. I dont' think a doctor would think it's worth it. Dwarfs sometimes get this done and it takes them a long time to grow just a little bit. Just exercise your back, arms, and chest muscles. Larger muscles will make it look like you have a broader chest without having to go through all the pain of surgery.