I don't have insight into the decision-making at WordPress, but they probably do this because they want to be compatible with as many web hosts as possible, and

I don't have insight into the decision-making at WordPress, but they probably do this because they want to be compatible with as many web hosts as possible, and so many super-cheap shared hosts just give you a public directory that you're supposed to dump everything in. It's possible to set it up properly on most of these hosts, but it's much more difficult, and if you ever have issues the support team says you're using a "non-supported configuration." At least WordPress lets you move your config file one level above the webroot and will find it automatically.