Mostly because if you're trying to do this kind of thing, you're trying to maximize activity per frontage so that there are enough people on the street to enliv

Mostly because if you're trying to do this kind of thing, you're trying to maximize activity per frontage so that there are enough people on the street to enliven it. If you have public right-of-way on both the front and back of the buildings, you've doubled the frontage without increasing the activity, so the place is only half as alive. In the very center, you might be able to do it; as it gets toward the edges, you need all the activity you can get.