Why not? It is objectively true that I was born on a certain day, and was given a certain name. The only way I know my birthdate and my name is by faith. There is no scientific way to prove either of these facts, nor are they mentioned in any history books that I am aware of - yet, they
are facts.
It’s the same with Church teachings. They are true whether anyone believes them, or not. The perpetual virginity of Mary is an existing fact that will continue to exist (that is to say, she will remain perpetually a virgin) whether
anyone believes in it or not. The Church teaches it, because it’s
objectively true - it’s not just an emotional opinion, or a myth that helps us make sense of something else. In the same way that we can believe our parents when they tell us our names and our birthdates, we can also believe the Catholic Church, when it teaches us about the truths of the Catholic faith.