It's really annoying when I try to add tags that obviously would fit to the game but twitch has predetermined ones for each game.

Even if you try to add an "open world" tag to a pretty obvious open world game twitch doesn't accept it. Instead the "Done" button transforms into a greyed out "Updating..." button and the window doesn't close.

Why can I even add these tags if twitch refuses to use the anyway?

Either the tags should be in the list at all depending on what game you choose or at least a warning message should pop up with something like "Please only use tags that fits to the chosen game".