Hidden information doesn't make a game more complicated. Rock Paper Scissors have hidden information but it's a very simple game for example. You can argue there is no hidden information in poker either if you think in terms of ranges. Your inputs are the public cards on the board and betting history - nothing hidden there. Your move requires a probability distribution across the whole range (all possible hands). Framed like that hidden information in poker disappears. The task is to just find the best distributions so the strategy is unexploitable - same as in chess (you need to play moves that won't lose and preferably win if the opponent makes a mistake).