1. There's a real social cost associated with constantly subjecting human reviewers to traumatizing content. It can lead to devastating mental health problems. (some relevant articles: https://www.theverge.com/2019/2/25/18229714/cognizant-facebo..., https://www.telegraph.co.uk/technology/2019/12/16/youtube-mo...)
2. Humans make mistakes as well, especially as volume of content increases and policies get more nuanced.
3. There's way more violating content then you could ever imagine. As just one example, YouTube reported having to remove >1.8 BILLION comments for spam or other policy violations in 2019 (https://transparencyreport.google.com/youtube-policy).
4. The large tech companies spend billions on human moderation as it is. YouTube alone for example has >10,000 full time human reviewers. (https://youtube.googleblog.com/2017/12/expanding-our-work-ag...)