Why is cc not allowed in the sims?

Basically, they don't SUPPORT CC.  They don't stop you from using it, and even make it easy for you, but they don't want to provide support for someone else's content.

I think if they supported CC and actively promoted it etc not only would they be seen as a company who wants the players to create the content for them, but also they would propably need to make sure the game works well with mods. By saying they don't support it they don't have to make the game actively work with mods or alter it with mods in mind. 

Also: liability. People make all kinds of mods into the game and is EA would condone mods they'd propably have to keep an eye on what kind of mods are actually being made. 
CC is allowed, but it's not super common for other game developers either to actively support and advocate mods made by the community. Sure some do, but EA is huge and the Sims is a huge franchise. 

The warning is there so that people don't ruin their games with mods, and then try to blame EA for something that they had nothing to do with. It's EA protecting themselves in case someone does mess up their game, because then they can simply point to those messages and be like "we warned you about mods and it's your own fault". Heck even Bethesda has warnings about using mods in their games, and they even have built in mod browsers in games like Skyrim and Fallout 4.

So it's not that they're telling you not to use mods, they're simply reminding you that you're responsible for anything that happens when using mods and that it's your own fault if things go wrong. If EA didn't want to allow mods in the first place, they would do like what Rockstar did and try to have mods taken down but instead they allow them and simply warn players that use them. I haven't played an updated PC version in years, but it used to be that you could even disable that warning message in the options.