WHY ARE PEOPLE LIKE THIS???

I am a college sopre in a BS/MD program with a highly ranked medical school. My college places a HUGE emphasis on service. I am participating in the Light the Night walk held by the Leukemia and Lymphoma Society soon. I have a personal goal of fundraising just $100 and have personally contributed $25, despite being the typical "poor college student". However, when asking my so called "friends" here for just $1-5 people refuse (with both the cash and online option). These girls will happilly spend money on Vera Bradley or iTunes songs, and the boys will happily buy knives(!), but they refuse to contribute to an amazing cause. I do not understand.