Because an occasional glitch when the client & server sync back up is acceptable in a game. Finding out that my order didn't actually go through is much worse. Especially since click button, see success, and close browser is an relatively common use case.

Consider these two scenarios.

1. SPA with asynchronous server communication. A button switches to a spinner the moment you click it, and spins until the update is safe at the server. Error messages can show up near the button, or in a toast.

2. LiveView where updates go via the server. The button shows no change (after recovering from submit "bounce" animation) until a response from the server has come back to you. To do anything better, you need to write it yourself, and now you're back in SPA world again.

There's a reason textarea input isn't sent to a server with the server updating the visible contents! Same thing applies to all aspects of the UX.

EDIT: https://dockyard.com/blogs/optimizing-user-experience-with-l... talks about this. That'll handle things like buttons being disabled while a request is in flight, but it won't e.g. immediately add new TODO items to the classic TODO list example.

That's a deliberate UI choice, though, and it doesn't always make sense in non-transactional workflows. It's easy to wait for Google Docs to say "Saved to Drive", and going to a new page to save a document would be really disruptive to your workflow, for example.