Why does the C++ STL not provide any “tree” containers?

There are two reasons you could want to use a tree:

You want to mirror the problem using a tree-like structure:
For this we have boost graph library

Or you want a container that has tree like access characteristics
For this we have

• std::map (and std::multimap)
• std::set (and std::multiset)

Basically the characteristics of these two containers is such that they practically have to be implemented using trees (though this is not actually a requirement).

See also this question:
C tree Implementation