Here's what I think, broadly:
(1) Lawyers have inflated egos, and like to use "impressive" vocabulary. They don't focus on writing skills, because they just figure they know it all already.
(2) Lawyers are afraid to commit to a definite position. If they do, there's less room to wiggle out of it. Their wishy-washy writing reflects this.
(3) If you have a 25-page limit for a brief, a client expects you to use it. Even if you can say what you need to say in ten pages, the client assumes you're not doing a stellar job if you don't use all 25.