Besides that, isn't it that much contemporary AT ammo is based on some sort of HEAT tech, meaning that the composition of the armor and/or its reactive capacities count for more than slope, and often more than even thickness per se. Perhaps its even that sloping the armor would reduce the reactive density, or sommat. Put another way, the boxy sort of shape a lot of contemporary tanks have comes from being lined with layers of replaceable and interchangeable blocks of composite/reactive armor; I can see that a lot of sloping would reduce both fungibility and resilience.