Catcalling as teasing is also low-risk, since you aren't offending someone you know, possibly making new enemies.

Also, since it's usually a male(s)-on-female occurrence, there's the superior physical strength of the harassers, often backed by strength in numbers. Suppose the conflict escalates; what could the victim possibly do to the harasser, that the harasser can't return with even greater force? Suppose she has a strong, visible negative reaction; you know what the catcallers will do? Laugh, ridicule and humiliate her. From their point of view, the behavior has all the benefits it could have, and none of the drawbacks. It's as low-risk an offense as you can possibly get.

Maybe that's where one can act to reduce instances of the behavior. Increase expected associated risk by a significant amount. Make it so that it no longer pays off. Unfortunately there seems to be no way to actually enforce a law or norm against street harassment, or to take any action that is both 1) a sufficiently strong deterrent and 2) within the bounds of legality and legitimate self-defense.