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The C++ standard doesn't specify how deque is implemented. It isn't required to allocate new space by allocating a new chunk and chaining it on to the previous ones, all that's required is that insertion at each end be amortized constant time.
So, while it's easy to see how to implement deque such that it gives the guarantee you want[*], that's not the only way to do it.
[*] Iterators have a reference to an element, plus a reference to the block it's in so that they can continue forward/back off the ends of the block when they reach them. Plus I suppose a reference to the deque itself, so that operator+ can be constant-time as expected for random-access iterators -- following a chain of links from block to block isn't good enough.
Because the standard says it can. It does not mandate that deque be implemented as a list of chunks. It mandates a particular interface with particular pre and post conditions and particular algorithmic complexity minimums.
Implementors are free to implement the thing in whatever way they choose, so long as it meets all of those requirements. A sensible implementation might use lists of chunks, or it might use some other technique with different trade-offs.
It's probably impossible to say that one technique is strictly better than another for all users in all situations. Which is why the standard gives implementors some freedom to choose.