Mainly because the bigger the group involved in bystanding behaviour the less likely it is that anyone will respond, each bystander believing essentially that "somebody" should take responsibility but feeling no sense of personal responsibility to do so. Its typical of situations where responsibility is felt to be diffuse.
Not buying that one? Well issues, sadly, of personal safety, liability and accountability, they result in either indecision or abdication of responsibility. The "I'm alright jack" reaction to anothers distress or need.
No good? Generalised cynicism, "This is a stunt and I will not act in anyway which could make me seem like a sucker".
An adult? Did you not realize that you're an adult at eighteen, dear? Certainly the Juniors and Seniors were more in the neighborhood of twenty-one, twenty-two, even twenty-three...it's not like you're defending junior high students. Hell, 100 years ago a fifteen year old could run an entire household.
I'm not buying your position at all, if any thing it's infantilizing young adults.
"Sentiment without action is the ruin of the soul." - Edward Abbey