Because most people would rather prepare for a product launch than go out and find actual users (because that's harder) and a product launch looks like productive work and talking to people is hard because they probably won't care about the product.<p>Good luck.
because not enough research was done prior - without research, talking to customers, getting feedback before launching, you will often end up with a product no one wants. for a product launch to be successful, it has to have a "wow" effect. having a "wow" effect means solving a problem that is really annoying with a great solution - this requires a complete understanding of the problem.