Because for the type of vapid people she attracts, image is everything. They know they're not friends, they're just close to her because they're popularity-hungry vultures a rank below her in the school's hierarchy. And her befriending a bottom rung loner, upsets this hierarchy and puts their positions in jeopardy. They're friends by proximity, so they need to maintain that proximity and scare off anyone who tries to get in the way.