Joseph Campbell devoted his entire career studying mythologies. Before he died he had written a sizable body of work in comparative mythology and comparative religion, which answers this question with a level of detail and breadth that is awe inspiring. I personally found his books absolutely fascinating.

I'm no historian but a simplified paraphrased summary might be: We are all human. We all experience the passage of time and the cycle of life. Our ancestors all struggled to overcome similar obstacles, faced similar fears, puzzled over the same mysteries, and strove to make sense out their place in the world which we all share. The commonality among our mythologies reflects this.