That's kind of like asking "why do people build choppers when they can just go buy a ready made motorcycle".
For me it's the fact that building then tinkering on a bike gives me something to do that I really enjoy. That and about 11 years ago I had a pretty good wreck on a motorcycle (which ironically enough I'd built out of a Honda and a Kawasaki) and almost lost my left arm. My wife said no more motorcycles and by the definition of the law a bike wit ha motor IS NOT a motorcycle!
Ouch! Stop hitting me dear!
Anyway if I was going to get a scooter it'd be a lot more then one of these $600 Chinese jobs. I've always been partial to
:edit: Scooters can be cool!