That’s because with managed clouds, you’re not actually paying for egress traffic when you pay for egress traffic. You’re paying your fair share of the incredibly expensive massive, distributed, low-latency, high-bandwidth, reliable network infrastructure used to glue the entire cloud together. The thing that can withstand massive DDoSes without breaking a sweat. It’s textbook cross subsidization.

CloudFlare doesn’t operate a massive cloud infrastructure so they don’t have the same cost structure.

Unfortunately the cloud providers don’t make this clear to the world, leading to a lot of grumbling from people who think bandwidth should be charged on a pure cost-plus model.