Why "don't" ?

[Mr.Lucky_One]

Why is there "don't" in the clause below?

She don't have to know.

And it's not only in this clause.
I heard many times people said "He/she/it don't..." in songs and movie.

[Gillnetter]

Why is there "don't" in the clause below? That is not a clause, it is a sentence.

She don't have to know.

And it's not only in this clause.
I heard many times people said "He/she/it don't..." in songs and movies.

"She doesn't have to know" is correct.
She does not have to know.

"She don't have to know" is incorrect, but is sometimes used in informal speech and writing. Unless this is used in a humorous way, a person using "don't" this way is considered to be uneducated.
She do not have to know.

[emsr2d2]

In songs, film and TV, you will hear a lot of non-standard English.