They have an exponential number of valid positions, that happen to surpass human abilities to abstract, memorize, and predict.
Chess is estimated to have 10⁴⁰ valid moves, which means not even everyone playing chess throughout all of history, have explored all of them. Like, a billion people playing 1 distinct move a second for 1400 years, would only reach about 10²⁰ moves.
They still can be trained, meaning one person can be way better than another… but a computer trained even more, can be even better… and yet the games surpass even current computers abilities to explore the full possibility space. Maybe quantum computers will be able to do that.