Why use React when you can use Unity, Unreal Engine, or Godot? Those engines are tailored for this, Javascript is not the language or tool for this type of project. Not sure if it is because you have not explored other tools/engines/languages?
Well, I was just demonstrating how popular metaverses like Decentraland are built. These are built in the browser and leverage WebGL instead of using Unity etc. I agree with you that this is not suited for big games which should probably rather use Unreal or Unity. However, this tutorial is about how to build it for the web with the tools that the web and its ecosystem has to offer and is targeted towards web developers rather than game developers.