The most prominent reason why there are so many religions in the world lies in the fact that all cultures are not the same. Cultures build religions based upon their own ideologies and philosophies. Like Christianity, many sub-groups appear and tend to mold a "main" religion into something which appeals to a religious group.
Outside of that, people simply need to feel religion for themselves. They search for the group which speaks to their emotional and spiritual needs. There is not one group which exists that can do that for everyone.
Many religions have loosely interpreted meanings, which leads to different interpretations by groups of people.
Christianity is one of the prime examples of this with sub-branches that includes Catholism, Baptism, etc
Many would say the Bible may be "incomplete" or people have misinterpreted the meaning.
This is a great question and I am sure that you will get many different responses. As you probably know, religion is one of those hot topics, which can cause a lot of emotions. In light of this, let me make a few observations.
First, you are correct, there are many religions around the world. Most of the world is religious. This is an important starting point, because in the West many educated people do not see this. Atheism is in the minority. Moreover, many intelligent people in the world believe in some divine being or beings.
Second, part of the reason why there are so many religions is rooted in culture. People from different places in the world have different convictions and beliefs. For example, if you live in the Middle East, you would realize that most people believe in one God. In India, most people have a polytheistic world view.
Third, human knowledge in many areas is limited. This is certainly the case when it comes to religion. The upshot of this is diversity of religions.
One reason is that religions tend to be divergent. In other words they split and divide into sub groups. Because the claims of religion are abstract and obscure, this leads to a wide variety of possible interpretations. So, for example, Judaism split when Jesus offered a new subset of claims and Christianity was born. Then a few hundred years later, Mohammed reformed the ideas again into Islam.
And Judaism, Christianity and Islam have thousands and thousands of subdivisions, sects, break-away groups, schisms etc.
Religion's basic topic is the nature and wishes of God. This is an eternal blank sheet of paper upon which we can all write our own ideas. Nobody can prove anyone wrong. For example, I could say that God is an alien super-computer that is totally undetectable but knows everything and can do anything. Prove me wrong. If I was persuasive enough, I could create a new religion on this idea (Scientology is a perfect example of a new religion sprouting out of one man's imagination.)