because anyone who has heard it is dead, and the others can't remember.
Though the song is said to be "irresistible", it's said that no one knows it because when you do, then something bad happens to you. Those who hear it end up dying or forgetting, showing how it has a heavy impact on individuals.
Right back where we had started
We started off with nothing, not trusting each other and being in the dark. The world we once had, the perfect and happy world, we made it into that, but then when we started getting too full of ourselves, we lost it. We had to build it again if we wanted to be happy, but it had to start with being able to live together again, tolerating each other.