Well, they aren't. And neither are all humans. If those old demon slaying families are important and have influence, they definitely don't want humans and demons to get along. If that happened, what use would demon slayers be? At most the country would need mere special officials who could get rid of dangerous individuals, demon or human alike. There would be no need for families who's only purpose is to wage war against the demons, probably eating a lot of resources to do it.