Why did Jesus start Catholicism?
The simple answer is that Jesus did not start Catholicism. Catholicism is a system of religion that gradually developed from the church started by Jesus, but Catholicism has long ago stopped resembling the original church.
Jesus stated, "And I also say to you that you are Peter, and on this rock I will build My church, and the gates of Hades shall not prevail against it" (Matthew 16:18). This occurred on the day of Pentecost as recorded in Acts 2. "And the Lord added to the church daily those who were being saved" (Acts 2:47).
Early on, the apostles warned that there would be a falling away from the truth. "Now the Spirit expressly says that in latter times some will depart from the faith, giving heed to deceiving spirits and doctrines of demons, speaking lies in hypocrisy, having their own conscience seared with a hot iron, forbidding to marry, and commanding to abstain from foods which God created to be received with thanksgiving by those who believe and know the truth" (I Timothy 4:1-3). Many recognize that hallmarks of Catholicism, especially Roman Catholicism is the forbiddance of priests, monks, and nuns from marriage and the forbidding of foods at certain times of the year. Another verse often cited is, "Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of sin is revealed, the son of perdition, who opposes and exalts himself above all that is called God or that is worshiped, so that he sits as God in the temple of God, showing himself that he is God" (II Thessalonians 2:3-4). Some see the Pope, ruling the Roman Catholic church, as a fulfillment of this prophecy. However, most Catholics would deny that the Pope is exalted as God. They only see the Pope as God's earthly representative.
The primary reason Catholicism left the original practices of the church Jesus founded is their belief that the church has the right to modify the commands of God. Catholics believe that truth resides in the church. The Bible is seen as only the starting point and that the church is able to modify it over time to fit current beliefs and cultures. With such a unstable foundation, it is little wonder that no longer resembles the first century church.