Because they sound good
Yes they do mate. A couple of years ago I took the "whispering death" stock pipe off my BMW K100RS SE SBS1 (what a fcuking mouthfull!) and replaced it with an uber-rare Luftmeister race pipe that I desperately wanted for my K100RT back in the 1990's but couldn't afford the $1200 US (plus shipping) asking price back then, on eBay for the pricely sum of $115 bucks plus shipping to Oz.
Apart from the extra 12 BHP that Luftmeister guaranteed back in the day, it now sounds like a motorcycle, not obnoxiously loud, just nice, and it howls when you give it some WOT. What's the point of riding a motorcycle if you can't hear it? You may as well just give in to the fun police and drive a fcuking Prius............