Why do AI models struggle with online hate speech detection?

Why do AI models struggle with online hate speech detection? Hate speech that once circulated in person now travels farther and faster via anonymous online accounts behind a screen. As the United Nations marks the International Day for Countering Hate Speech on June 18, UN Secretary-General Antonio Guterres has warned that social platforms are amplifying the threat. With artificial intelligence (AI) increasingly tasked with detecting and removing hate speech online, Al Jazeera looks at where these systems fall short compared with human judgement. How is hate speech defined?