This is one reason for so many splinter denominations, of course - this freedom of thought and belief. But for many Christians it is more important to exercise these freedoms rather than yield to the conformity required by Catholicism, especially when they find so many traditions to doubt, so many scriptural verses that seem to contradict other verses, so much in the Bible that they can't agree with. Take the first 11 chapters of Genesis as examples of stories that surely are legends and not history, or verses like Ex. 22: 18 and 20. Did God order Saul to commit genocide against the Amalekites? Did God arrange at the request of Joshua to have the sun stand still? And we could go on and on.
For those who love liturgy, the Episcopalians and Lutherans provide it. For those more interested in focusing on inspiring sermons and less concerned about ritual, other denominations provide such worship. The Methodists are sort-of in between, depending upon the local pastor and/or parish. For those who are ready to believe that the Catholic church is the one true church, that the Holy Spirit keeps it from error, that all of its doctrines are infallible - Catholicism is a fitting choice.
I've often asked myself: has religion as a whole done more good or more harm? It has done much good in 'good works' - hospitals, education, charity work. It also provides important communities where individuals can enjoy the benefits of an extended family. It helps undergird people with faith when they confront major challenges. But religion also has promoted bigotry and narrow-mindedness, yes even wars. It has closed millions of minds, stifled many discussions, undermined many relationships, fostered many myths as truth, etc. I recalk when it the CC considered it a major sin to attend a Protestant church, even to attend a baptism, wedding, or funeral! Such stupidity. Thank God for John XXIII and Vatican II!
I do not criticize Unitarianism. I admire it. But it is not for most people. It focuses mainly upon the intellect, which is fine. But most human beings have an instinctive desire for something more. They want more than a totally rational religion. This vast universe is an amazing creation, and some spirit of awe, a deep reverence for God - the heart and mind bend in this direction. But most mainline Protestants prefer doctrinal democracy - work out your own theology within a generally Christian context - rather than accept traditions and beliefs so often inherited from centuries ago when there was so much less knowledge than we have today, before modern telescopes and microscopes, when Christians believed that the earth was flat, that heaven was up in the sky somewhere, when we all had guardian angels, when there were demigods (saints) in heaven to help us whether we were soldiers or saddle-makers, when disease was usually caused by demons - and we could go on.