Why does it feel like nepotism/legacy is a big problem in admissions?

I got waitlisted and I don't know why... I had a 1550 SAT, 4.66 Weighted GPA in my freshman year at my school, and a fire essay (in my opinion). I don't think my teachers hated me and I GOT WAITLISTED!!! One of my friends had a 17 ACT (WTF!?) but had 2 siblings and a parent who went to IMSA and he got accepted? Not to be mean but he was kinda a bum at our school, and someone he got chosen over me? Is IMSA not care about prestige because I don't know why they wouldn't chose a F student over someone with a 1550 (98th percentile) SAT score.