Why is the committee so reluctant to add new features to the language itself instead of stuffing them into the STL?

This goes eg for smart pointers but also modern container-related stuff, or things like std::optional. A lot of this could have been added more elegantly with new syntax or keyword additions, surely? And that is not even mentioning keywords like static already overloaded with multiple meanings. I feel like eg C# was pretty aggressive with this and ultimately the syntax benefited from it.

One argument to this I have read is the usual "backwards compatibility" etc but that never made much sense to me - if that is a concern then you would probably just stick with the older version you need anyway?

I just feel like the syntax is becoming increasingly cluttered because they seem deathly afraid to touch any of the core language.