Because they give a lot away and post blog posts attacking AWS folks think they are cheap. But a lot of their free stuff may be in the marketing budget. The issue here is:
1) Their paid offerings for their core product, CDN, does not have a public pricing guide. That's a BAD sign if they claim to be competing on price. So we can pretty safely assume their paid service bandwidth charges are NOT the 80X cheaper than amazon despite their claims here.
2) They hype their bandwidth alliance. I tried following this up on Azure, it's basically all call a sales person / maybe you'll get a deal type stuff. Smoke and mirrors again.
3) If you actually try to use their CDN to serve something high bandwidth (games / software updates / video streaming) that is going to violate some type of clause which says that the free CDN is not REALLY unlimited.
4) Financially they spend a mind boggling amount on sales costs. Between sales costs and cost of sales I think they are negative relative to revenue.
BTW - If you try any of the other "free" and "unlimited" providers (for $5/month etc) you will also find out once you try to move some real data volumes (video etc) - it's all a lie over and over.
Same things with geo issues. Cloudflare claims to have "one" price globally. These are almost alwyas false claims. It's easy to test - setup a stream into a high cost area, stream a bunch of video there - you'll usually get a call over some tricky TOS issue they pull out.
In short, if you want pay and forget, there is a reason the big cloud players have a business still.