Consoles don’t do that, they have a set resolution and try to use that even if the monitor supports higher res, the reason is that a console has a limited hardware and knows its limitations, that way they can ensure the best user experience. The Steam Deck works primarily as a console, but without the usual closed off limitations so you can change the resolution if you know that the specific game you’re trying to play will support it, but as a general the deck plays it safe and uses the same resolution it would use for the integrated screen.
Also you need to think on the cluster fuck it would be if you’re playing the game in 4k then unplug your deck and now you have a 4k game downscaled to fit a 1280x800 screen.