If you don't know what a content delivery network is, here : https://www.cloudflare.com/learning/cdn/what-is-a-cdn/

A CND is very costly to run in an effective way. And because it is an intermediary server between the user and content server, the market is already pretty full. So competing with the CDN giants is practically impossible in a decentralised manner.

Because of what a CDN does (cache website elements closer to the user, protect the website against ddos...), it cannot be a cheap weak server, or it's the one which will get overwhelmed by the ddos, or even the users.

Another limiting factor is that in decentralisation, that means different companies, and so many separate plans to pay, which is just impossible for a company.

If it was decentralized, a company would have to go and pay 100 different companies (which is more expensive, du to the server costs and each companies having their own staff to may (even if it's just 1 person per company)) just to offer a quick access to the users around the world, which is just impossible.