I'm only speculating, but the reason is likely because language servers are not written in JS. I assume electron apps can spin up arbitrary processes, allowing for a language servers to be written in anything. It'd be neat to see these language servers compiled to WASM and run as a web worker, although I suspect vscode isn't setup for that.

I think Codespaces is able to pull it off because the language servers, and vscode itself, is really running on Github's backends. They've set up the frontend to connect to a remote host.

I handle this by firing up a Fargate instance with a Docker container pre-configured for VSCode Remote SSH. Then once you connect to it, you can install whatever language server you want assuming it's not baked into the Docker image. An EFS mount persists my sources across runs too, and I can scale it up to whatever size Fargate instance my development of the day needs.

Would be cool to tie it into this.

Thats the trade off. If you have everything supported upfront then the loading performance would be abysmal and it would take 2 minutes to load the entire app on a desktop machine. This is perfect for browsing through code.