Historically, people have been killed for way less than metaphors.
If you built a power structure on top of a religion, or if you think that preserving respect to certain ideas leads to a more harmonious society in general, you can very easily prosecute people that undermine them, whether you believe them as real or not.
It’s hard to imagine who Augustine wrote his Confessions to or on whose behalf Galileo was killed, if all these people were just talking about metaphors.