Question: Why are there different religions?
Answer: I suppose the simple answer is that people are sinners. And because people are sinners they will find any possible permutation of religious elements that make sense to them and that gets them off the hook from worshiping the true God.
In Romans 1:18-26 Paul asserts that the truth about who God is, His eternal power and “godhead”, is known to everyone. It is made clear through creation so that no one has an excuse to say, “I couldn’t find God.” However, as Paul points out, we suppress that truth. We want to live our lives the way we want to, not at the dictation of our Creator, so we bend the truth in order to make religion palatable to us. We can’t escape that there is this religious reality…God…but we can distort the truth enough to feel less pressure to submit to Him.
Consequently we shape God in our own image and when you do that you have to have a different form in which to pour that image and, viola!, you have a new religion. It also becomes a pathway for some to power as they define what the religion looks like and what its doctrines are and as they dictate the practice of it. Eventually as your family and maybe your whole society embraces this it becomes a tradition that you cannot deny and still be viewed as part of your community.
Now, to be fair, we need to distinguish between different religions and different expressions within the same religion. If you compare Christianity, Islam and Hinduism, for example, you will find that each is mutually exclusive of the other. You cannot consistently hold to one and hold to the others as well. But within Christianity there are varieties of expression of the same basic doctrines, and the same is true for Hinduism and Islam. There is Catholicism, Protestantism and Orthodoxy within Christianity but they hold many of the same basic beliefs. Some of this is tied to differences in personality or culture, some to variations as to which doctrinal beliefs are most important.
The possibility always exists, however, that some version of worship that was compatible within the larger religion can move toward “heresy” and become incompatible with the beliefs and practices of the major religion. Sometimes they will acknowledge that and declare they are no longer a part of that religion, but sometimes they want to stay in the fold even though they have contradicted its major tenets.
All this to say, there are different religions, religions that depart from the revealed truth about God, because people are sinful and they yet cannot refrain from being religious and salving their conscience by having some form of worship.