Vehicular cycling at least puts you where drivers expect other vehicles to be coming from. It's a good choice in many situations. Being in pedestrian areas, riding as far right as possible (area-specific mandate), or in a bike lane makes you practically invisible in many cases.
Cycling specific infrastructure can do a lot, but it can never be a solution for all destinations because of limitations enforced by pre-existing infrastructure.
my knee-jerk reaction is: and because streets aren't just for cars!
But more pragmatically, vehicular cycling is often the best choice because of visibility and predictability, and also as regards driver awareness and expectation: it at least seems safer if drivers are accustomed to sharing the road than if they never have to.
Cycling specific infrastructure can do a lot, but it can never be a solution for all destinations because of limitations enforced by pre-existing infrastructure.