Crazy girls can't keep relationships. From the time they're 16, they bounce from dude to dude, while others who are more sane tend to find longer-term relationships.
Being perpetually single means way more chances to suck new D, and the reinforcement they get from practice with new people/compliments means they get better at it. And more quickies means more opportunity to get faster and better at it.
So chances are, if a girl can S a D like a pro, she's crazy and the relationship won't last, if only because her cawklust for the strange cannot be quenched.
OR she's a fat girl, just lips and titties and dudes always choose the BJ over sex because they don't want to see her naked/risk preggo/be there longer than 8 minutes anyway. 95% of men will take a quick string-less BJ from a decent looking fattie... way fewer will get naked and spend the night and leave their seed where turkey baster can find it.