Have you ever tried to read a math textbook that cherishes being short and concise? They're nigh unreadable unless you already know everything in them.

That's not entirely true -- Melrose's book on Geometric Scattering Theory, Serre's book on Lie Groups and Algebras, Spivak's book on Calculus on Manifolds, and so on.

I think the phenomena you're pointing to is closer to the observation that the traits that make one a good mathematician are mostly orthogonal to the traits that make one a good writer.