This is a great question. There is no one reason for mythologies. So, let me name a few of them.
First, there is something in humans that need to explain the world around them. In light of this impulse, we can say that the Greeks created myths as a way to make sense of the world.
Second, myths also have a social function. In other words, people do not only want to explain the origins of the world, but also their own place in the world and their interaction with one another. This also created myths.
Third, when these myths took root, people, then, began to tweak these myths further and interpret them. This created more myths. This is why there are so many variations of myth in the Greek world. I will attach one of the most enduring work on this topic by E. Hamilton.