Because it is not just a case of snapping people out in the street doing not much. Ideally there should be a story or an artistic aspect to the picture. After all if it was just a case of snapping anyone doing anything then everyone would be able to do street.
However it is much more than that, its a case of finding that scene and waiting for the “actors” to appear. Or spotting someone and waiting for things to develop. For instance one day while out doing street I saw in the Flinders street station a man carrying a cross. He was back in the shadow, however I deliberately waited until he had partially moved into the sunlit area. Now the picture represents a much more involved story. He is now representing the notion of religious enlightenment , the moving from the dark to light, of finding god. It also fits with his one ungloved hand versus the black gloved hand. Making the image much more interesting and dynamic. The waiting well that is what Henry Cartier-Bresson was all about with his “decisive moment”.
Sometime of course things unfold so quickly that it is difficult to find that decisive moment. Which is why sometimes it is better to find a scene and wait for the “actor/s” to enter and complete the story.