There isn't any particular reason why Amazon might not decide one day to copy Cloudflare as one of their services.
And then all clients of Cloudflare that are also AWS clients will switch to AWS for the same service, same cost, but one less headache.
On the other hand, Cloudflare is unable to copy AWS business model.
So, revised title: "AWS is playing chess, Cloudflare is playing Go on a board and time borrowed from Amazon"