3) Cost. There's a perfectly good and expensive pipe already on the bike that complies with regulations, and is built and tuned for that bike. Harley gained a reputation for changing a bike from a toy, into an attitude, and people were willing to waste big bucks to be thought to have that attitude. This effect has since spread to other manufacturers, who are perfectly willing to accept people's hard earned cash.
If you're racing the bike, and you're good, then it makes sense to go for performance gains. If you're just tracking the bike, then you won't even be using the extra performance.