Normally, questions are only formed with do-support in English if the main verb is not an auxiliary or the copula (be).
This also goes for questions with why:
You go with me -> You don't go with me.
Why do(n’t) you go with me?
You are a man -> Are you a man?
Why are(n’t) you a man?
So if your question is simply asking what the reason is that someone is X, then you cannot use do after why: you use simple auxiliary inversion instead.
However, the construction “why don't you (just)…” has another, slightly different meaning, too. It can be used as a rhetorical question basically equivalent to a rather crude and exasperated-sounding imperative:
Why don't you (just) shut up?
Why don't you (just) leave me to do my work and worry about your own, eh?
In this usage, there is no real question. The asker doesn't care what the reason behind X not shutting up or not leaving him to do his work are—he just wants X to do it. Basically, the meaning is:
Leave me to do my work and worry about your own!
In this usage, the main verb is implicitly interpreted as an action verb, rather than as a stative verb, and the inherently stative verb be ends up forming a very close connection with its predicate, yielding a kind of pseudo-action verb meaning “to do all the things that are normally associated with X”, rather than a stative verb meaning “to have the identity of X”. And this verb-predicate does take do-support:
Why don't you (just) be quiet for a moment?
Why don't you (just) be a man about it and stop whining?
These semantically mean:
Be quiet (= do what's associated with being quiet)!
Be a man (= do what's associated with being a man)!
– but because be X is so tightly connected as a unit, it takes do-support when forming these rhetorical why-questions.