And here the actual reasons why it’s so complicated (warning: this is extremely oversimplified):
Collabora exposes what feels like a hundred endpoints and relies on WOPI (a protocol originally designed to make Microsoft Office available in the browser) because, at its core, it is not a true web application but essentially a desktop application (LibreOffice in this case) that is serevd through a web server.
The server does most of the heavy lifting, the browser displays the rendered output and the user’s input is sent back. In return, you theoretically get the full feature set of a desktop suite in your browser. In practice, however, some features are still missing, probably due to performance constraints.
Native web apps like Google Docs, OnlyOffice, or Excalidraw (the Nextcloud Whiteboard app) take a very different approach, as most of the application logic is loaded directly into the browser itself, so that only the document data needs to sync with the server.
In short: WOPI-based apps behave more like remote desktop solutions than modern web applications. 