Basically because many practice eisegesis rather than exegesis. Others just want validation and not truth. Some look for justification for what they want to practice. They gather teachers who will tickle their ears. With still others, it is a matter of: "I never would have seen it if I hadn't believed it [already]!" Many teach doctrines which find support, not in scripture but, rather, in paganism.
Still other professed Christians match the description found in Jesus' illustration at Matthew 13:24-43. This illustration is profound and its meaning is even more profound. Most people do not realize that Jesus prophesied long ago that there would be counterfeit Christians in among the true Christians. Have you ever seen a counterfeit bill? Amazingly, it looks like the real thing. Close examination, however, reveals its counterfeit characteristics. So it is with true Christianity. It ought to match what is found in the Bible. The Bible is the touchstone of Christianity and true Christianity is a pillar and support of the truth found in the Bible even as the apostle Paul told Timothy. And true Christians would speak in agreement, even as the apostle Paul wrote to the Corinthian congregation. How can Christians be poles apart in teachings and behavior, disagree with each other, form literally thousands of denominations and yet be true followers of Christ? It is impossible.
When you look for an explanation for all these denominations, look to the people who are in them, not the Bible. If they do not match the teachings, doctrines and conduct in God's word, how can they possibly practice the Christianity they profess?
Hannah J Paul