I dunno. Maybe because it makes a hell of a lot more sense than the extreme fundy freakshow who constantly and mindlessly denounces anything which doesn't fit their narrow definition of Christianity? You mean like,
• Pope Leo XIII, Satis Cognitum (# 9): "The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium." Satis Cognitum (# 9): June 29, 1896: http://www.vatican.va/holy_father/leo_xiii/encyclicals/documents/hf_l-xiii_enc_29061896_satis-cognitum_en.html • Pope Boniface VIII, Unam Sanctam:
“We declare, say, define, and pronounce [ex cathedra] that it is absolutely necessary for the salvation of every human creature to be subject to the Roman Pontiff.”
"If, therefore, the Greeks or others say that they are not committed to Peter and to his successors, they necessarily say that they are not of the sheep of Christ, since the Lord says that there is only one fold and one shepherd (Jn.10:16). Whoever, therefore, resists this authority, resists the command of God Himself. " — Pope Boniface VIII, Unam Sanctam (Promulgated November 18, 1302) http://www.fordham.edu/halsall/source/b8-unam.html
• Fifth Lateran Council: Moreover, since subjection to the Roman pontiff is necessary for salvation for all Christ's faithful, as we are taught by the testimony of both sacred scripture and the holy fathers, and as is declared by the constitution of pope Boniface VIII of happy memory, also our predecessor, which begins Unam sanctam, we therefore...renew and give our approval to that constitution... Fifth Lateran CouncilSession 11, 19 December 1516, http://www.piar.hu/councils/ecum18.htm
• Pope Pius XI, Mortalium Animos: Furthermore, in this one Church of Christ no man can be or remain who does not accept, recognize and obey the authority and supremacy of Peter and his legitimate successors. Did not the ancestors of those who are now entangled in the errors of Photius [the eastern “Orthodox” schismatics] and the reformers, obey the Bishop of Rome, the chief shepherd of souls?...Let none delude himself with obstinate wrangling. For life and salvation are here concerned...” Pope Pius XI, Mortalium Animos, PTC:873) The Promotion of True Religious Unity), 11, Encyclical promulgated on January 6, 1928, #11; http://www.vatican.va/holy_father/pius_xi/encyclicals/documents/hf_p-xi_enc_19280106_mortalium-animos_en.html
• Pope Eugene IV and the Council of Florence: "The sacrosanct Roman Church...firmly believes, professes, and proclaims that..not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life but will depart `into everlasting fire...unless before the end of life the same have been added to the flock; and that..no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ, can be saved, unless he has remained in the bosom and unity of the Catholic Church.” — Pope Eugene IV and the Council of Florence (Seventeenth Ecumenical Council), Cantate Domino, Bull promulgated on February 4, 1441 (Florentine style), [considered infallible by some]