Deque - how come "reserve" doesn't exist?

Increasing the size of a std::vector can be costly. When a vector outgrows its reserved space, the entire contents of the vector must be copied (or moved) to a larger reserve.

It is specifically because std::vector resizing can be costly that vector::reserve() exists. reserve() can prepare a std::vector to anticipate reaching a certain size without exceeding its capacity.

Conversely, a deque can always add more memory without needing to relocate the existing elements. If a std::deque could reserve() memory, there would be little to no noticeable benefit.

For vector and string, reserved space prevents later insertions at the end (up to the capacity) from invalidating iterators and references to earlier elements, by ensuring that elements don't need to be copied/moved. This relocation may also be costly.

With deque and list, earlier references are never invalidated by insertions at the end, and elements aren't moved, so the need to reserve capacity does not arise.

You might think that with vector and string, reserving space also guarantees that later insertions will not throw an exception (unless a constructor throws), since there's no need to allocate memory. You might think that the same guarantee would be useful for other sequences, and hence deque::reserve would have a possible use. There is in fact no such guarantee for vector and string, although in most (all?) implementations it's true. So this is not the intended purpose of reserve.

Quoting from C++ Reference

As opposed to std::vector, the elements of a deque are not stored contiguously: typical implementations use a sequence of individually allocated fixed-size arrays.
The storage of a deque is automatically expanded and contracted as needed. Expansion of a deque is cheaper than the expansion of a std::vector because it does not involve copying of the existing elements to a new memory location.

Deque can allocate new memory anywhere it wants and just point to it, unlike vectors which require a continuous block of memory to hold all their elements.