Why are there so many Christian denominations?

Jesus often prayer that his followers would have such a radical unity that would be a testimony to the world. However, when we look at the Christian body today, it seems as though they divide up into hundreds of different groups, churches, and denominations. For those who are not Christians, it seems confusing. Why can’t they agree on anything? Why are there sometimes four different churches on the same street? Even for believers, the question often arises: Why are there so many Christian denominations?