Maybe it's because they don't have all that sheet metal to express their style... but people who ride 2-wheel transportation seem to like dressing up.
You've got your "bikers"... you know, the ones who ride Harleys and Harley-lookalikes. They don't necessarily ride their "bikes" much; in fact it seems like a lot of 'em are used as driveway ornaments, or to ride to the Harley shop or a bar. That's about it. But when they do ride, they love to dress up in their biker costumes - you know, sinister black leather, "do rag," boots. Like Marlon in that movie. Helmet? Hahahahaha! Good one!
You've also got your punks on the crotch rockets. Some of 'em dress in the leather racing suits. More of 'em dress in cutoffs, a sleeveless T-shirt, and flip-flops, and sometimes with a gal on the back, similarly attired. I'm not sure if that's a deliberate costume choice, or just what they happen to be wearing... but I see it often enough that I'm suspicious they intentionally dress that way.
And then take a look at bicycle riders. Motorists like to poke fun and/or scorn at their spandex gear. (Perhaps there's a bit of jealousy there, I dont' know... or perhaps they don't appreciate the subtle advantages of spandex.) Some cyclists like to wear the "team kit," as worn by cyclists who race in Europe... even when they're just puttin' up and down the Greenbelt! I'm not sure what that's about. Is it a "let's pretend" thing (like the leather-clad "bikers")... or is it like my Raiders t-shirt? (I like the Raiders... do some cyclists follow Astana or Radio Shack in the same way?) (Radio Shack! huh-huh!)
(I'm not much of an expert. About the only recurring "costume" I wear when riding motorcycle OR bicycle is the self-explanatory hi-viz vest and/or jacket. Oh - and brain-bucket.)