Except that, in at least some circles, "y'all" can be used to refer to one person.
(The answer one does not want to hear, at least in Georgia, is "is Pepsi OK?")
To take someone down a peg if they seem full of themselves, you can use the majestic singular satirically. As in: I just got a fancy new car! Gosh, did y’all get seat warmers with it too?
It's literally a contraction for "you all". That doesn't make sense for talking to one person.
And yes, this is a real expression that is used in parts of the US, not pure comedy.