Standard web APIs are a lot like kernel-level OS features. They have a ton of inertia to them, so they arguably should be limited to stuff that is infeasible in "userland". (There are additional reasons for that in the case of OSs, but that's the relevant one here).

If you had such a high-level API baked in, it would get leapfrogged by some library project within a few years, and then we'd be stuck with yet another API to support that no one actually uses.