I am not a writer, but I always wondered why characters tend to not talk using discourse markers, specifically cognitive markers to create flow in the dialogue. There aren't any "um", "so", "uh", pauses, etc. Realistically, people use discourse markers almost every utterance. Even someone confident on what they have to say still uses it, especially if they're still formulating their thoughts. Wouldn't use of it make dialogue much more believable?
One of a writer's primary goals is to make their writing flow, and sound pleasant and rhythmic to the ear. And frankly, interrupting words like "er", "uh", "um", and so on are distracting. They jolt the rhythm of a sentence and throw off the reader if used in excess. While it would technically be more realistic and plausible to include them, it can make a passage difficult to read. Imagine, for example, if characters in a movie were constantly puttering around and being unsure of their dialogue:
"The, um, the, uh, the missile is, er..."
You would think, from watching this kind of performance, that the actors had forgotten their lines, or not prepared properly for their part. It's not the kind of thing that really sells the character, and it can detract from their likability and charisma on the screen.
In short, there's a fine line between "naturalistic" and "annoying" when it comes to using stammer words (as I like to call them). It can quickly cross the line from a quirky character trait into excessive, and while it would strictly speaking be more realistic to have characters be unsure of their dialogue, good character acting is characterized by strong line delivery and confidence, whether that be in movies or in literature. So usually, when trying to write strong, charismatic and likable characters, it is good practice to minimize the use of stammer words, using them only when it might be helpful to characterize someone as hesitant or shy.
They're not used normally because there's a difference between writing dialogue that sounds and feels realistic and naturalistic and transcribing dialogue that is real.
There's an old rule of thumb that if you want someone to sound like an idiot, quote exactly what they said, every sound that comes out of their mouth exactly the way they said it. Every pause for thought, every slight mispronunciation, every dropped phoneme. It's things we ignore in spoken dialogue, but written, it's glaring and distracting and can even make the most eloquent person sound like a bumbling half-wit.
Here's an example:
"I can't really tell you, but I...suspect it was decided quite early on...with cons-- with our consult our consultation, but Shawn Matthias would have would have his views on that. Um it just seems to be right I I can't imagine anyone now, uh, playing Hirst other than you. Uh, and... I think I could have had a crack at it but I I I but that's all. I couldn't have done it."
The obviously stumbling and clearly nervous person I just transcribed is Sir Ian McKellen having a conversation with Sir Patrick Stewart.
Now, if I wasn't simply transcribing the dialogue, this is how I would have written it that, ironically, more accurately describes how he comes across in the video:
"I can't really tell you, but I suspect it was decided quite early on with our consultation, but Shawn Matthias would have his views on that." Sir Ian paused for thought. "It just seems to be right. I can't imagine anyone now playing Hirst other than you. I think I could have had a crack at it, but that's all. I couldn't have done it."
It's trivial to find other examples for yourself. Simply do what I did; take a recording of someone and transcribe exactly what they said. Not what you heard, what they said. Humans mentally filter out those pauses and duplications and all the other little quicks which people do all the time because they aren't usually important to the message. The brain will focus on the overall meaning, not details. It doesn't work that way when you're reading.
I think you've mixed up your terms, and are really asking about verbal pauses, which communicate that the speaker is still formulating their discourse or choosing next words, and not discourse markers, which are used to frame concepts or signal the start or end of a specific argument.
Dialogue in fiction minimizes verbal pauses because it is usually kind of annoying. It can be used sparingly to communicate qualities about a character, but when over-used, as in used like we use it in everyday communication, it really robs moments of intensity and passion.
This is because the best written dialogue is almost always more representative of the best of conversation and not a conversation itself. If a real conversation was communicated in a scene, it would be really boring -- usually. In the best dialogue, characters often talk past one another because it is an effective way for the author to communicate the character's inner emotional and intellectual state while also advancing the story.
In casual conversation, the human brain tends to filter out most discourse markers which are interjected between words. The ones which largely stick out tend to be the "umm..." pause typically accompanied by a non-verbal cue as well.
The thought process for reading dialogue is markedly different and as such this auto-filter mechanism isn't applicable. Therefore, the markers simply stick out and impede the dialogue rather than supporting it.