I think that's just a lie motorbikers tell themselves. Exhausts are pointed backwards. Cars are built so well today that they barely let in any noise. I was surprised so many times while driving a car, because a motorbike was suddenly overtaking me out of nowhere. I can't even hear a motorbike passing me when I am on a motorbike myself, up to the point when the other rider is right next to me.

They maybe help pedestrians and cyclists, but cars, trucks, other bikes etc. don't really hear motorbikes.

You can only hear them at slow speed. Anything above 50kph and the air noise make it impossible to hear. Motorcycles are invisible because a lot people don't take driving seriously and don't check their mirrors/blind spots.

The only time I got into an accident was at a red light. I was stopped in front of a car, which was also stopped. The light went green and the car rear ended me, this was mid day, 100% visibility, my bike was bright red, you can't fix stupid.

The noise from motorcycles is projected behind the bike, so it's not terribly effective at alerting unaware people to their presences. And, as most people who've tried to determine the direction an ambulance is approaching from will tell you: it's really hard to locate objects based on sound emissions alone from within a car cabin because of the acoustics of a cabin end up reflecting the audio all around the driver.

If you want to stand out on a motorcycle, use your lights and wear bright, reflective clothing. Even then, you have to accept that you'll never have the presence of an automobile, so drive accordingly.