Fastly also has more "points of presence" than AWS/GCP
Fastly has deployments in 17 north american cities. AWS is in 5.
They're pretty different strategies. For every massive AWS datacenter on a continent, fastly has at least 3 tiny ones -- inevitably closer to your customers, meaning lower RTT.
AWS, GCP, and Azure have CDNs as well, but those are more typically used by existing customers to front things running there.