IIRC the main reason it isn't enabled by default is because >=1080p is only available via DASH. Normally Invidious can just point the client to fetch videos from Google's servers, but for technical reasons DASH requires the Invidious instance to act as a proxy (the client asks the instance for video data, then the instance fetches it from Google and sends it to the client). The net result is that watching 1080p streams requires much more bandwidth from the server.