Somebody noticed (https://www.openstreetmap.org/user/StephaneP/diary/390290) that OSM isn't even consistent about which datum it uses. (Discussed in HN at the time: https://news.ycombinator.com/item?id=20460596 ). (Though to be fair, the datum error is probably smaller than e.g. GPS error in most cases.)

As for the "why" bit, the answer is probably "store and report whatever the user reports".