Because filing the form S-1 comes quite a bit before the actual start of trading. S-1 is a prospectus, lets investors familiarize themselves with the offering.
I'd also add that this is just NASDAQ. There's a lot of other exchanges across the world.
Though, obviously, offerings in other (foreign) markets won't necessarily file S-1s because S-1 is a concept of the United States' Security and Exchange Commission (SEC).