Because the doctrines of the Catholic Church are all consistent with Scripture. Whereas, reformed Christian doctrine, such as Scripture alone, contradicts Scripture.
In fact, and with all due respect, any doctrine, Jewish, Christian or otherwise, which contradicts Catholic Teaching also contradicts Scripture (I’m including the Tanach in the word Scripture.)
I know that sounds provocative, but we believe that the Catholic Church knows the Jewish Scriptures better than the Jews themselves.
St. John 5:37 And the Father himself who hath sent me, hath given testimony of me: neither have you heard his voice at any time, nor seen his shape. 38 And you have not his word abiding in you: for whom he hath sent, him you believe not. 39 Search the scriptures, for you think in them to have life everlasting; and the same are they that give testimony of me. 40 And you will not come to me that you may have life.
Sincerely,
De Maria