“Amazon Web Services continues to dominate a global cloud-infrastructure services industry that hit almost $70 billion last year, grabbing a market share equivalent to that of the next four public cloud providers combined.”
Let that roll around in your head for a while. AWS alone sold as much cloud service as Microsoft, Google, IBM, and Alibaba combined.
Where there is more market share there is more opportunity which means more jobs. It stands to reason that AWS environments need four times as many people as any of the other.