Grammar

Most people write the way they think, which is to say in half-formed impulses that lean on context and guesswork. Speech forgives that. Writing does not.

They rush. They imitate whatever loose style surrounds them. They avoid precision because precision forces you to decide what you actually mean. Most people would rather not look directly at their own intent. It is easier to pad a sentence than to own a point.

There is also the small matter of schooling. Grammar is taught as something else to regurgitate rather than a tool that can be used to create clarity. You end up with adults that know fragments of terminology but cannot stitch a thought together, either in their brains or in writing.

None of this makes them bad. Just untalented and unskilled. Writing well requires effort. Most people conserve effort wherever they can.