The reason is that the Catholic Church (not any other church) is God’s body on earth.
You know that when you are thinking something, you can get your hands to type it out, and your hands can get your computer to send it to us.
In the same way, when God wants to send us a message, His hands that are typing out the message are, the Catholic Church. The Catholic Church cannot do anything without God willing it to, just as in the same way your hands can’t act independently of you - if your hands (the Catholic Church) are to type something out, then your mind (God) has to be thinking that thought, and asking your hands (the Church) to type it out.
To be away from God’s body (the Catholic Church) is to be away from God. Can those who are deliberately and purposely away from God, be saved? Only with great difficulty, I think.