Deliberately making sentences harder to parse (by, e.g., bad grammar, bad spelling, lack of capitalization, lack of vowels, leet speak) is annoying.

Non-capitalization may not seem like a huge decrease in readability, but every little bit counts.

It makes visually scanning the sentences harder, capital letter is bigger than period. I mean, sure, it is readable, but I do prefer regular text, you know, like in a book or newspaper.

something that does meaningfully affect readability is WRITING IN ALL CAPS. you're almost guaranteed to have encountered the visually distinct shapes of lowercased words thousands more times than you've encountered the shapes of their uppercased counterparts

It slows down my reading speed. I A/B tested my page reading speed on my kindle a few years ago

Slop content and grammatical presentation are separate concepts. Grammatical correctness and creative sentence structure are also separate. One can have an interesting structure whilst still being correct, therein lies the creativity.