Since the book of Revelation appears in Catholic Bibles, why do Christians claim it's about Catholicism ending?

It is about all religions ending. They have been an enemy of God. Christendom will be destroyed first because they were entrusted with the truth of God and they polluted it with pagan worship and celebrations. Then the other religions will fall too. The only one remaining will be the true religion. Of course Revelation is in the Catholic Bible but they do not think they are a false religion. They do not think it applies to them. But read the description of Babylon the great how she got drunk on the blood of the saints. That fits the Catholic religion the best. Considering the burning at the stake of all people who tried to read the Bible for themselves or translate it so the average person could read it. The Catholic Religion has the bloodiest history of any so called Christian religion. Right up to today. In foreign countries they will turn in other Christians to the authorities if they are not Catholics. Do some research. You will find a lot of information about them. Most of the people do not even know what they have done. But they still stick by them today in spite of all the abuse scandals and the theft of church money by many of the Priests.