Sadly, most cradle Catholics do not know how to explain their faith biblically to a Protestant or even a non believer. I would bet dollars to donuts that converts from Protestantism know more biblically about the Catholic faith then cradle Catholics themselves. That is just a gut feeling.
Also when one explains their faith to another, they have to not just know it reasonably well, but defend it from honest criticism. This takes more than just going in and out the doors to mass every week. It takes personal study at home. Just how many Catholics from all walks of life really take the time to really do this at home, especially when working all day, raising kids and wanting to watch TV or do other recreational activities?