XMPP, Mastodon, etc. aside the Web became centralized largely because we who built for the Web let it happen. The Web used to be a lot more decentralized where we, the people who built stuff for the Web, wrote in our blogs and helped each other learn this thing we were at the same time developing standards for to improve. This all ended when almost all of us converged on Twitter.
The social network of the future: No ads, no corporate surveillance, ethical design, and decentralization! Own your data with Mastodon!