I agree. That comment is meant to come off like a compliment, but it’s an insult. “How are you still single?” Implies that “you” are a catch, too attractive, etc, to still be not chosen by someone. Then the “still single” implies that being single is a bad thing to be, that one must be in a relationship or we are thought of as a non-productive person not engaging in what is expected in life.

I wish I could think of a witty come back for you to say to them next time they say it. How about, “How are you still stupid?” No, jk! Lol