This is not about whether people can make mods for the game. It's about us hosting those mods or allowing our site to be used to advertise them. I'd be interested to see if there are official places where you can download mods for those games (excluding AC as their section was closed). I'm not interested to go searching for them at the moment as I have work to do, but if you could link to them that would be helpful.
As mentioned above, there are two problems.
1) People copying or converting vehicles made for other games then hosting or linking to them here. In that case LFS site can be seen to support copyright infringements. This cannot be allowed and would require strict moderation. This is one of the reasons given for closing the car mods section on Assetto Corsa.
2) People creating models of real cars. This is obviously fine for their own use but there may be questions here. Supposing we have cars made by various famous manufacturers listed on our site, either as forum threads or even in some kind of future in-game mods download system. There is a possibility this can be seen as us profiting from the names, reputation and IP of those companies. Or maybe it really is fine for users to create such content and upload it. We can't take the risk unless this is established.