The number 6 is quite nice. It has a lot of factors and is also the smallest perfect number. Unsurprisingly, it shows up everywhere in a number of religions. People might easily have started with the number 6 and designed the star to go along with it. While it was harder to travel thousands of years ago, people did and it only takes one to bring back a design like this.
We can trace every script in Europe, Africa and Asia back to just three, one from Mesopotamia (3400 BC), another from Egypt (3250 BC) and the third from China (1200 BC). If writing was able to make it's way to almost every single culture on three continents, a 6 pointed star certainly can.