A manually approved system where you (as the game developer) open yourself up to criticism if something offensive to someone, somewhere slips through the net and goes viral? I wouldn't want that job. It's easier to just not allow it, and so here we are.
What's Minecraft's rating? Think I can't make offensive content on Minecraft? I don't see your logic here.
Realistically other large game developers have these systems in place working right now and have for some time.
Armored Core, Battlefield, Call of Duty, Forza.. to name a few.
If you count the addition of mods in the new Bethesda games which open up the public to literally add anything including artwork then Skyrim and Fallout 4 are also included here, granted they are single player however their content is shareable on Xbox Live, PSN Network, and Steam for everyone.
Saying FDev can't do it seems like a cop out, I believe they are fully capable wether you have faith they can do it correctly or not.