(1) Because it's the speed limit.
(2) "Motorists traveling to and from Dulles International Airport on business are permitted to use I-66 inside the Beltway during HOV hours." Cited
here.
(3) People are stupid.
(4) Have you ever spent time in a minivan full of 10-year olds? I'd want to get to my destination faster, too.
(5) Motorcycles are cool.
(6) People that work in Fairfax, but live in Loudoun do.