Partly because...
They fund research; they have a huge reputation and can entice the best of the best to work there; they get lots of funding from lots of sources. My good friend, Annie, had brain surgery in Joplin, MO by a Johns Hopkins trained neurosurgeon. He was one of the best and his father had been a neurosurgeon at Johns Hopkins for many many years. This guy consulted with his father and they invented a surgery just for Annie that saved her life. Eventually, he returned to Johns Hopkins to practice.
They teach the best of the best and keep the best of the best. Their reputation is well-earned.