Comment by tgsovlerkhgsel

Because bandwidth (and static serving) is dirt cheap, presumably especially so for someone like Cloudflare. Hetzner used to charge ~$1.20 per TB beyond the generous included allowances.

Most sites will have a hard time getting anywhere close to that and the ones that do will likely at some point want more advanced features than the free packages offer (or get force-upsold, see e.g. https://news.ycombinator.com/item?id=42713451).

Once people are in the Cloudflare ecosystem, they're much more likely to upgrade and start using additional services, or recommend Cloudflare to their employer.