if you started before the game jam starts, you wouldn’t know what theme and could potentially make the wrong game
You can't make a game you don't know the theme for, but you can prep. Like batman, with preptime he beats everyone. You can make input, UI, character, camera, etc controllers and other helper logic and focus on making the actual gameplay for the jam. It's completely unrealistic to do all that in 48hs AND the actual game. Tools =/= game.