Because itch.io doesn't support in-game purchases. That's why the game comes with a steam key, so you can use the key on steam and make ingame purchases from there.
Because I have all the hats on my PC and my sister doesn't (on her PC). We both use the itch.io version. I remember back when I downloaded mine it was like "among-us-windows-all-hats.zip" or something like that but my sisters copy isn't.
