Having been neighbors with a lot of these guys, it has been repeatedly explained to me that theh make their bikes louder to be heard for safety. The assumption here is car drivers are so engaged with their music/podcasts/whatever turned up that they don't pay attention to the traffic around them. They also tend to not look when switching lanes or, gods forbid, use their turn signal to show intent.
While this sounds like a reasonable argument, I think it's a load of bullpucky. Motorcycles are loud by default, and using aftermarket parts to make them louder is just part of the self-affirming statement of ego. These guys need you to know how big and bad they believe themselves to be that they'll make other sacrifices in their lives to afford projecting that image.
It's really all very silly, to be honest.