Anyone can write what we call dialogue, but writing good dialogue is no easy task. It takes time and practice to develop a quality ear.
In a screenplay, dialogue is conversation, but conversation in everyday life is definitely not dialogue. Real talk is boring.
If you read a transcription of a real conversation - even if the subject matter is controversial and full of passionate opinions - it’s completely absurd. This real talk is disjointed, long winded, redundant, unfocused, and often just too much information.
So in writing good dialogue, it’s never about capturing truth or reality: how we really talk. Realistic dialogue only gives a flavor of reality. It is artful deception. That isn’t to say that the screenwriter doesn’t write dialogue that reads like real speech. Not at all. It must feel and sound believable, but the irony is that believable dialogue doesn’t exist in real talk.