I recently spoke about "Is the God of Muhammad the Father of Jesus?" (You can get the video here). One of the things I mentioned was the idea that if we truly believe that there is only one God, that person is the only being that a human can be attempting to worship.
The monotheistic belief says that people can not worship other Gods because those other Gods do not exist. Which is how we ended up resolving our primary question. If there is only one being, then contradictory statements about that being cannot both be true.
However, this concept is the beginning to an answer of why there are world religions in general. In Romans the Bible says that since the beginning of the world God has been revealing himself to us through what has been made so that we have no excuse (Romans 1:19). That idea, called general revelation if you are ever on jeopardy, is saying that through sunsets, roses, and giraffes, God has been calling to us telling us about who he is.
That is why when we look up to the stars, all of humanity has begun to long for connection with the God who made those stars. When we have looked at a sunset we have longed to connected to the artist who painted it in the first place. That is where the religious impulse coes from. That is why there are so many religions.
I think general revelation is also the reason that so many peoples, especially in the ancient world, developed polytheistic or animistic religions. Because God was calling out to them through creation, the mistook the voice for the person and began to worship the creation for the person.
That is our primary task as believers. Not to argue theological minutiae, but to live out the revelation of the one God all humanity has longed for since the first time they gazed upon the night sky.
If you would like to read more about the Islam and Christianity piece, I recommend the chapter of the same title as my sermon in the book: Theology in the Context of World Christianity: How the Global Church Is Influencing the Way We Think about and Discuss Theology