T-14 Armata

[Husar]

Read as "a blast", advances in tank armour since the end of WWII gradually made tanks invulnerable to HE shells not coming from a Battleship, so we moved to SABOT which is a penetrator and it became more important to make the front of the tank and the turret armour as thick as possible to stop said SABOT rather than to create a nice round shell which was more conventionally strong. Also, we've got better at welding things together, so nice round turrets are no longer as important.

Amazing. I was surprised you brought up blasts because the question was specifically about KE penetrators.
Or so I assumed since he said projectile weapons. If by projectile all missiles etc. are included it gets a little more complicated.
Though even in WW2 most shots were probably AP rounds and artillery can still hurt tanks quite a bit, at least by hitting the optics or with modern AT rounds that penetrate the roof with HEAT much like cluster bomb cluster munitions.

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.

Well, not for tank vs tanks, where it's mostly about giving the KE (kinetic energy) penetrator as much of that kinetic energy to make it punch right through the armor. For missiles, rocket launchers and the abovementioned cluster bombs and artillery shells you often get HEAT or CE (chemical energy), yes. Whether the armor is always easily replaceable I'm not so sure, if you mean the addon blocks they put on the outside, that's usually in the areas which are historically only protected by relatively thin armor and vulnerable to insurgents coming from all sides. so they put cages in the back as spaced armor of sorts and reactive armor blocks on the sides for additional protection. Sloping the sides would not only make the tanks much wider or smaller on the inside but also be a potential weakspot in tight streets with enemies on the roofs, who could now not only hit the roof very well, but also go for those sloped sides which would look rather flat from slightly above.
The boxy german tanks of WW2 were also boxy because it gave them more space inside and it was easier to stuff all the equipment into a box than to put it into a wedge.