Why doesn’t `std::initializer_list` provide a subscript operator?

According to Bjarne Stroustrup in Section 17.3.4.2 (p. 497) of The C++ Programming Language, 4th Edition:

Unfortunately, initializer_list doesn’t provide subscripting.

No further reason is given.

My guess is that it’s one of these reasons:

1. it’s an omission, or
2. because the initializer_list class is implemented with an array and you’d have to do bounds checking to provide safe access, it could more easily be used unsafely if that interface was provided, or
3. to be consistent with the std algorithms iteration paradigm, or
4. because initializer_lists are, by their nature, ad-hoc, there’s more room for error by addressing them directly

2 and 4 sound kind of weak. as does 3. My money’s on 1.