People don't use proper grammar when speaking. True, that.
This is why, when editing a writer's manuscript, I correct little if any of the grammar in dialogue. What I will do is highlight the grammatically incorrect phrase and explain in a margin note what the grammatically proper phrasing would be. The author then has the option to leave the dialogue as originally written, to accept the grammatically correct version, or to revise.
When it comes to reading dialogue, I expect better standards from characters representing higher social strata of civilization, as they're more likely to have received extensive instruction in manners, deportment, academics, etc. In short, they have an image of social superiority to uphold and the oftentimes stuffy phrasing and cadence of correct grammar supports that image. Lower caste or class individuals may be forgiven their broad slang, crude idioms, and sloppy sentence construction in dialogue.
One of my favorite movies is Auntie Mame, starring Rosalind Russell. In the scene where Mame's beloved nephew hires a ghostwriter, Brian O'Bannion, for her, the gentleman immediately flatters her with a short sentence: "I was asked to drop by to meet the fabulous Mrs. Burnside. You are she, of course."
Combined with a look of admiration and casual elegance of appearance, the ghostwriter's exacting grammar hints at superiority: a superior mind, a superior man. He's a fortune hunting cad, of course
Another favorite movie, Last of the Mohicans, has a scene in which Nathaniel Poe and company come across a burned-out cabin with its former inhabitants slain. Nathaniel, Uncas, and Chingachgook proceed, but Cora Munro objects to leaving the dead without a proper Christian burial, even though they're strangers. Nathaniel replies, "They are not strangers .... and they stay as they lay!"
The grammarian in me winces at that, wanting to correct lay to lie. The bodies lie (present tense) on the ground. However, use of the incorrect verb testifies to the character's lack of education, if not his honor and savvy.
Those two examples show how grammar can be used to play upon social perception and conceal the true character of the person speaking such dialogue.
Using correct grammar in speech isn't necessarily easy nor does it come naturally. It can help clarify meaning, but often gets short shrift in the interests of economy of expression or brevity. More than most prose, dialogue embedded within the story receives leeway, wiggle room, and lenience because people don't converse in correct grammar.