Because the response they are receiving when they 'advocate for the better of their demographic (and the type of response that is turning them sour/disillusioned) is:
"Let's say everything you said was true" - iteria (needlessly implying it's a lie/not happening)
"someone is providing an easy scapegoat for their own problems" - tayo42 (implying what they are experience/feeling isn't real/valid. This takes some temporary bullshit that they are feeling and ends up ossifying it into ugliness/a horrible position/a shit worldview, and drives them into horrible online communities designed to take advantage of this all and feeding them into the trash right pipeline).
I'm not justifying it, I hate that I'm losing my boy into it, but I can only push so much without it pushing him harder into 'one side doesn't validate my feelings so I'm going the other way'. I think a little empathy from his peers would have gone a long way. Instead he was ostracized by his peers when he moved back home because he went through a crappy situations that seriously impacted his life and wanted someone to share a little empathy with him, because he was a white male asking for that empathy.