A makes an ignorant comment.

B says that is so ignorant.

A genuinely asks why.

B actually takes the time to explain that if they have to explain it to A then A is already a lost cause not worth the explanation. If B is being extra snarky, will further state that A is not worth explaining anything to. For all that time typing B could have actually provided an explanation, even if it was just a link.

Why does B even go through the motions of addressing A at all?