"why don't you?" is a question.
"why you don't" is a command.
Why you don't? is not a correct question. Also it is not a command.
Why can be used to give a suggestion or piece of advice eg.
'Why don't you sell your car by advertising it in the paper?'
I think 'why you don't' is a variation on this usage eg
'Why you don't sell that car I'll never know'
i dont no
i dont no
i dont think there is a difference
we dont know
i dont know
i!dont!no
dont care
i!dont!no
dont know dont care
i dont know lol
i dont really know
I dont no sir