I dont know why tf so many of them feel so compelled to belittle others

Bullying often comes from insecurity. In the first place she believes (probably taught by parents) that unattractive people are unworthy and subhuman. Combine that with emotional abuse that undermines her self-esteem, and you get someone who desperately needs affirmation that they are part of the in-group and not one of the undesirables. Putting someone else down gives a momentary feeling of being above the undesirable group, therefore among the desirables, even though she knows deep-down that social acceptance can only come from others.