Loud horns don't overcome even louder sound systems and then there's the issue with deaf people (yeah, that's something that's happened to me before - why didn't they hear my horn? Oh, they were literally deaf!)

Most motorcycle crashes involving other vehicles are a result of drivers crossing the rider's path, e.g. making a left-hand turn in front of a rider who's going straight. Honking your horn in such a scenario can actually be a bad thing because most driver's response to hearing a horn is to stop. Now the driver is completely blocking the path and not moving. It's typically better to let the driver proceed and swerve to the left to avoid collision.

That's why most riders consider horns to be useless.