can you define "constant"
Your physical servers should have similar issues if you put a CDN in front unless the physical server is able to achieve a 100% uptime (100% * 3 9s = 3 9s). Or you don’t have a CDN but can be trivially knocked offline by the tiniest botnet (or even hitting hacker news front page)
And yes, I know that there's sites that need the scale of an operation like Cloudflare or AWS. But 99.9(...)% of pages don't, and people should start realizing that.