Why use 'I are' 'You is'?

I am and you are are the Standard English forms. In other dialects, the same form is used for all persons and numbers, so it is unlikely that a speaker of a nonstandard dialect would use both I are and you is. It would normally be are throughout or is throughout. As the sociolinguist Peter Trudgill says here:

Standard English has irregular forms of the verb to be both in the present tense (am, is, are) and in the past (was, were). Many nonstandard dialects have the same form for all persons, such as I be, you be, he be, we be, they be, and I were, you were, he were, we were, they were.