Your wall outlet is AC but a battery is DC, so a pure wire setup is not so much a safety thing as it is just incompatible. A good fire starter.

So you'll have to convert it, which makes for a big, bulky plug.

On top of that, you'll need prior knowledge of the battery layout, including whether they're parallel or serial. Usually parallel, but not a guarantee.

All of this isn't insurmountable, but is enough to make it not the norm. They do, in fact, already exist