I think it's a combination of:
1. Ragebait being so useful at generating engagement that it's become a standard form of human interaction
2. The Internet making people feel safe from physical repercussions which makes people feel comfortable with treating others badly
3. Internet communities quickly becoming echo chambers where you're forced to pick a side if you want a sense of belonging
So we've been programmed and manipulated to be angry, to be tribal, and to act without fear of retaliation, and all for what? For ads and followers.