I don't know about Swift, but in C++, the complexity serves at least three purposes:

1. Backwards compatibility, in particular syntax-wise. New language-level functionality is introduced without changing existing syntax, but by exploiting what had been mal-formed instructions.

2. Catering to the principle of "you don't pay for what you don't use" - and that means that the built-ins are rather spartan, and for convenience you have to build up complex structures of code yourself.

3. A multi-paradigmatic approach and multiple, sometimes conflicting, usage scenarios for features (which detractors might call "can't make up your mind" or "design by committee").

The crazy thing is that over the years, the added complexity makes the code for many tasks simpler than it used to be. It may involve a lot of complexity in libraries and under-the-hood, but paradoxically, and for the lay users, C++ can be said to have gotten simpler. Until you have to go down the rabbit hole of course.