Because the Bible contains allegory, poetry, prose, historical genres based on at least three different ancient languages that underwent morphological evolution during the writing of the books of the Bible, and literary particularities that make a literal reading of an always imperfect translation by a layperson ambiguous at best.
Your average Catholic does not simply pick up the Bible and extrapolate doctrines & conclusions. This is absolutely inimical to being a Catholic.
Infallibly condemned in Unigenitus by Pope Clement XI:
80. The reading of Sacred Scripture is for all.
81. The sacred obscurity of the Word of God is no reason for the laity to dispense themselves from reading it.
Declared and condemned as false, captious, evil-sounding, offensive to pious ears, scandalous, pernicious, rash, injurious to the Church and her practice, insulting not only to the Church but also the secular powers seditious, impious, blasphemous, suspected of heresy, and smacking of heresy itself, and, besides, favoring heretics and heresies, and also schisms, erroneous, close to heresy, many times condemned, and finally heretical, clearly renewing many heresies respectively and most especially those which are contained in the infamous propositions of Jansen, and indeed accepted in that sense in which these have been condemned.