Well he discovered the cycle by noticing trends in history before he realised he was looking at an actual mathematically precise global cycle. What you're saying is kind of like saying the number Pi itself is arbitrary. You're entering into the realm of asking why the universe is constructed in ways that continually manifest out of mathematical constants like the number Pi. If you're asking specifically why it's Pi * 1000, and not Pi * 10000 or Pi * 100, the answer to that is that the cycle is fractal. If you continuously multiply or divide the timespan of 3141 days by pi you will get more numbers that he has found correlating trends for as well (approximate 8.6 year cycles within 27 year cycles within 86 year cycles, etc.) If you appreciate the fact that the universe is cyclical and fractal in it's very nature, the idea that the collective activity and behaviour of the entire human race would manifest itself in a pattern doesn't really seem that far fetched.