Why does std::vector require its elements to be assignable?

[kempofighter]

The only operation I can think of that would require the elements to be assignable is the vector's own assignment operator, but why would an operation like push_back() require invoking the assignment operator of its elements? Even in the case of a reallocation, can't the vector use placement new to copy-construct the new elements from the old ones?

Placement new? How is that relevant to the question? The object still has to be copied one way or the other and this is done through either the assignment operator or copy constructor.