It's essentially the same issue as with various types of… bad content… on other platforms like YouTube. There's so much uploaded all the time that nothing is reviewed by humans until it's already out there. It's just that with porn sites, sexual content isn't forbidden by the terms of service, so taking down nonconsensual sexual content requires disputing the nonconsensual part, not the sexual part. And obviously that's a much more sensitive problem than the things e.g. YouTube generally gets criticism for.

It's a tricky issue. I think the measures they've announced to verify identities when uploading are probably the biggest thing they could do without a complete change of business model.