Because people don't want to look at other people's unusual niche kinks and try to identify if it contains children or other non-consenting participants as a part of their employment. And then try to explain to the people about why the unusual niche is against the rules when they receive a complaint about its removal. Amongst so many other reasons.

Programs and apps on the internet are not simple things and user-generated content can be a very malicious and dangerous thing to have a copy of. The legal risk alone is not worth it, let alone the effort required to enable a slightly easier masturbatory experience for total strangers.