You only ever hear about people who run away from cops badly. Motorcycles are usually long gone before the police have a chance to call a helicopter, which they would be able to outrun anyways if they were on a straight road.
To be clear though I don't think that's the reason loudness isn't enforced. I imagine it more has to do with the fact that they aren't watching for it since they don't have any way to easily test it like speed.