probably because motorbike riders know how to ride - they do comprehensive training to be able to use the road legally and not ride like an idiot.
i personally think everyone on a bike should do a motorbike test. having done it myself - i ride my bike on the road like i do my motorbike, and have never had a single issue. i honestly cant think of one, and thats since 2001.
if anyone is actually serious about being a good rider its a good investment, ive seen loads of riders who think they know what theyre doing. but they really dont.
Still get gits buzzing you in cars.