Because FOSS shouldn’t add burdens. You publish your work and let everyone else use it. That shouldn’t add extra obligations on you. Usually, you’d also write some docs - after all, without them nobody will know how to use your program, so why bother publishing - but it shouldn’t be an obligation. Make it easy for people to open up their code without this attaching strings.
Documentation is nice, but it’s kind of different thing that open source: a program can be open and undocumented, or closed but well documented - and I don’t see why we’d want it different for models.
The biggest issue is that there isn’t a universal agreement on what causes harm. There is agreement on the basics - murder, violence, etc - but they’re already illegal anyways, no need to ban them by license.