What's the main cause?

This centralized state is caused by many factors. The most prominent ones stem primarily from core design decisions of the World Wide Web.

Designed by programmers, for programmers

In an age where even basic filesystem structure knowledge has been abstracted away by mobile operating systems one cannot assume that the end user is able to set up a dedicated HTTP server on their own, much less acquire an open port and a domain name.

Designed for mainframes

The WWW was designed in a time when a mainframe server was necessary to do any kind of non-trivial computations while the end users' terminals were delegated to only downloading and previewing content. It was not designed with edge computing in mind.

Designed without mobile internet in mind

Today everyone is always connected, and the primary way of accessing the internet are mobile phones.

Designed for static content delivery

Dynamic content is a CGI-based hack on top of existing old HTTP infrastructure, even though it forms the basis of current web interaction.