Volume and competition. There are a handful of companies making parts for that Harley, versus untold thousands making parts for that Chevy. BTW, this isn't just a motorcycle phenomenon, it applies to most any recreational product I can think of. Ask the guys with boats how they like buying parts for those "holes in the water into which you throw money".
Jerry
And yes, they charge that much because they know we're stupid enough to pay the freight. A better question would be why do we tolerate the lousy stock parts that cause us to have to spend an additional $600 and up for a decent seat, or $400 and up for shocks that actually work? Compare the price you pay for that Harley with all it's cheap ass parts with what you pay for a 5 passenger automobile, and then try to tell yourself they aren't screwing you.