I wonder how much of this has to do with the medium of communication. People often have wonderful clarity when speaking but then muddle things up horribly when they write/create decks/etc.
I previously worked with a very smart and capable woman who was in charge of a client-facing department at an agency I was part of. She would ask me to review a presentation she had put together (often sales-focused, but not always), and I'd go through and at the end I'd often find myself saying, "You know, I'm finding this is (muddled | too much information | unclear). What exactly are you trying to say?"
And then she'd give me a succinct, spoken paragraph that completely nailed it. And I would say, "Okay, write that down and present that!"
I thought about this a fair bit because it was such an effective approach. It bears noting that, AFAIK, different parts of the brain are used for written communication as opposed to spoken communication. For instance it is possible to acquire a brain injury that prevents speech but not writing or vice versa. So although I think part of this issue is psychological (people get less clear, e.g. unnecessarily formal or stilted, when writing), I think another part of it has to do with the way people think.
I think a good technique that helps with this is having someone verbally ask you pointed questions ("what does this do?", "why should I care?", "how is it different?") while being recorded, and then take your spoken answers and use those in your written materials.