My understanding is that Fastly has a lot more edge locations than AWS, so using their image CDN results in lower latencies for many users than they could achieve with Cloudfront alone.
When I asked for detail about why they don't use Anycast, the Cloudfront engineering team basically said their customers care more about uptime than latency and that full Anycast was too sketchy. Apparently amazon.com disagrees, at least. I'm also happy getting much lower first page view latency out of Cloudflare.