Because instead of your eye naturally flowing down the text you get a sentence or two and then have to jump down to find the next sentence or two which while remembering the context. They aren’t contiguous at all.

Also because of the constraints of tweet length the sentences are chopped short rather than being whatever their “natural” length, short or long, would be.

The point of writing as the GP suggested is to minimize the friction between written text and comprehension/reasoning. Twitter is designed exactly against that, so the result is a bunch of slogans or tiny sound bites.