Because the composite armor inside are flat sheets. It’s just easier to have blocky shapes to contain these.
When armor was just made out of homogeneous steel, casting was an option. You’d design it to have the most slippery angles for shells to bounce off of.
Near the end of the WW2 saw 2 new types of penetrators being developed. 1) Kinetic penetrator : tungsten darts (later, depleted uranium darts). These can go deep.
2) The Germans built some 8 million shaped charges that tore up steel like cardboard.
During the 1970s, the engineers in UK developed Chobham armor. Instead of one single piece, it would have many sheets of armor incased in rubbery plastic.
(Soviet composite above)
These can defeat both types. The jet from a shaped charge would be warped because of the rubber. Even if it penetrates one layer, the next layer would warp and redirect the jet, making it useless.
Rubber can break the kinetic penetrator also. Not by itself; but by spreading metal sheets apart. (to one below that says “No rubber”: 2 sheets without rubber allow the penetrator to sail through).
Rubber when exposed to 1000 degree hot rod, would bulge up. That would push the metal plates away from each other. A penetrator caught between the two expanding plates break into pieces.
In the West, one or a few of sheets are made out of very hard ceramic (like boron or silicon carbide) or dense depleted uranium. That could blunt the force quite a bit, and the subsequent layers can break the penetrator apart.
Because of these flat internal layers, modern armors are blocky. (Even if they look vertical, often what’s inside are slanted.)
If you look at modern armor on top, you can see the sides are not thick. Much like WW2 German tanks, most of the modern armor is concentrated on the front. (You can see the cheek armor containing the composite plate.)
Japanese Type 90 above, South Korean K-2 Black Panther below.
Because of the weight, most tanks cannot have thick turret sides, however (Leopard 2 below).
The turret of Challenger 2 has one of the beefiest sides. It it still may not protect from a perpendicular shot from the side. But the cone of protection could be as wide as 90 degrees. (This also makes it one of the heaviest tanks too. Composite armors are not light).
The Russian T-90 uses an interesting design of making the cheek armor wider. This arc also looks to be about 60 degrees (30 degree to one side). You can’t even see the side of the turret unless it’s turned more than 30 degrees.
Europe has a lot of old bridges. Some of them could collapse under 62 tons of Challenger 2. That can limit the usefulness of heavier tanks. (This one does say 12t, what where they thinking driving a 50 ton M60?)