Why don't road bikes use v-brakes?
It may be tradition, and it may be aerodynamics.   Note that some roadsters do... Touring bikes have gradually been switching from the traditional cantilevers to V-Brakes.  It's for tire clearance mostly.
"Good" caliper roadster brakes are very powerful; A set of high-end Shimano jobs (105 or above) will lock your wheel and they are routinely used by pro road racers.
They have a minimal aero profile and contribute little drag.  They also look good on a sleek roadster.
Also, you don't have to weld or incorporate lugs onto your snazzy carbon-fiber fork legs and seat-stays, which results in a "cleaner" appearance and less weight as well; high-end roadsters are all about weight, as you likely know.