That's not entirely true. Sloped armor does come with a practical real world space penalty. It is a tradeoff and not one to be disregarded. The commonly cited three aspects of a tank, speed, armor and firepower are overly simplistic and don't actually reflect reality. The reality is studies conducted during and after WW2 showed that in almost all cases the first tank to fire would win. The first that could identify the target, aim its weapons and fire had the overwhelming advantage. That means crew performance and situational awareness is probably the most important aspect of a tank.
Case in point, compare the combat performance of early and late T-34s. Early production T-34s fared extremely poorly against their German opponents. Not because the Germans had better guns or armor, in Barbarossa we are talking about Pz3's after all, but because the Germans could see the Russians before the Russians could see them. The Germans had a three man turret, a commander's cupola, decent optics that were clear and had good light accumulation and radios. All of those aspects contributed to raising crew efficiency. The early model T-34s had cramped 2 man turrets, optics that actually seemed better on paper in terms of fov but weren't as bright and usually lacked radios. The later T-34s, especially the 85mm model fixed most of these problems. It had a proper turret. The crew had radios and they had more useful combat optics.
Back to sloped armor. The T-34's armor was actually too sloped if you can believe it. Not only did it have a sloped front but it also had sloped sides. This helped with protection, but the Soviets found in practice that sloping your sides brought too many problems. It limited storage volume for things like ammunition and fuel tanks and the Christie suspension also exacerbated this. In future designs like the T-54/55 they went to straight sides with torsion bar suspension.