On the Assymetry of "is" and "are"

Rappers, different stripes of yokels, and drivers of 30 MPG smart cars frequently replace "are" with "is" in dialogue, resulting in constructions like "they is" (e.g., "Rodney and Mikey? They is out back, Mama.") or "we is too" and so on. Good stuff. But rarely do we see the reverse: "He isn't back yet" is not often converted into "He aren't back yet". "She is" doesn't end up "She are". So there's a kind of assymetry to the improper use of these words, and I think it has not been explained fully or even fairly recognized. What makes "is" an attractive substitution for "are", but not vice versa?