I guess it’s because I know so many people who claim to be Catholic but are really not even Christians.
I used to be one. Now I follow Christ with all my heart. And I don’t believe he ever lived in a palace in Italy.
If someone is a Hasidic Jew, they're Jewish. If you join Sunni Islam, you're a Muslim. Given that Catholicism is a subset of Christianity, wouldn't someone who belongs to the Catholic Church be a Christian by definition?