I wonder how bad writing works

Sometimes in fanfic I see people who just can't write dialogue. They have the characters talking in giant run-on sentences without commas.

"Good morning ma'am sorry to bother you but we're with the police and we need to ask you a few questions."

"Oh well come in then pardon the mess but I have three kids and just got them off to school and haven't had time to clean up."

It's like that for the whole fic, which is unfortunate because usually the premise of the story is interesting, but I just can't hear the characters in these run-on sentences.

This has me wondering how exactly the author ends up in that place. They are exposed to proper dialogue construction in their recreational reading (and I know they read recreationally because these are book fandoms, plus the majority of fic in the fandoms uses properly-constructed dialogue), and yet in their own writing they land on these unpunctuated run-on streams of sentences.

When translators misfire on tone, they tend to err on the side of sounding like themselves instead of sounding like the source text. I'm not enough into fiction writing to know for certain, but it would stand to reason that in fiction people would make the similar error of sounding like themselves instead of sounding like the character. I know that in my own many epicly failed attempts at fiction, the problem was that all the characters sound like me.

But, I don't know about anyone else, but I think in sentences, with clauses and punctuation and everything. They aren't always impeccably structured - this post is a representative braindump - but they are sentences. That's just the way my thoughts arrive once they have manifested themselves into words.

So does this mean there are people walking around out there who don't think in sentences? Do their brains give them an epic spew of words to which they must consciously and manually add punctuation?