The limitation isn’t there, it’s deliberate. If you look when you connect to a monitor, the resolutions you can select for a game are resolutions detected by the Deck to be supported by the monitor. The Deck deliberately doesn’t go for the highest supported resolution by Default, I imagine this is because the Deck’s hardware is optimized for lower-resolution play, which is why it usually just stops at 1280x720p cuz the deck itself is 1280x800p which is pretty close.