Plus, as if tensor fields on Lorentz manifolds weren't already complicated enough, physicists aren't happy until they can write down some differential equations. So not only are you doing calculus, you're doing it on curved manifolds, with complicated tensor objects, in the context of partial differential equations, which - in the case of general relativity - are non-linear. It's okay to admit that all of this is a bit hard. Hell, as the article points out, Einstein himself had trouble understanding them.