"Some people" is too broad. Some people believe airplane contrails are a government program to control the population. Some people believe they will be reincarnated as insects. I can't give you a reason why "some people" don't believe what I believe. More to the point, I am begining to wonder if you actually have SPECIFIC examples that other Christians believe something significanctly different than what I believe. If you do, please offer them. For example, exactly who believes Christ wasn't present in actual physical form at the Last Supper?
I CAN tell you why I believe what I believe. I believe that the Bible is God's Word. I believe it is THE source of information on His plan for His children. I believe that He communicates with me directly through the Holy Spirit, and that with His help, even someone like me can understand His will and His word. Through the course of years of personal study and devotion, I have witnessed Him working directly in my life. I have actually felt His presence. With the birth of my two children, I have been given a clearer insight into the depth of His love for His children. Traveling throughout the world, I have witnessed his incredible work. Looking at a picture of an eyeball, I can see the miracle of His design. My relationship with God is DEEPLY personal. That is by His design. He sent the Holy Spirit to ME. Not just to my pastor or bishop or even the Pope. He never said I needed HUMAN approved doctrine to understand His will. He told me to pray directly to HIM. If you find comfort in the Catholic Doctrine that is fine. God approaches each of us differently and communicates with us according to our needs. But I highly recommend you back up your faith in Catholic Doctrine with personal study of God's Word. Don't take ANY MAN's word for it. Ask God. He will talk to you. He is FAR more capable than any HUMAN of communicating His Word. Certainly you must believe that.