When we are using "they" as a gender-neutral pronoun, why aren't we saying "is"?

I don’t see any movement toward number neutrality. Surely whatever pronoun we use, we know when we’re talking about one person. So why the plural verb? We are broadening the meaning of “they,” not narrowing the meaning of “are.”

I could speak of one person as “they is” much more readily than I can say “they are” of one human unit. Either way bothers me, because of the meaning of the words themselves, but a singular verb would bother me less.