Like in the games of chess, Go (game), and many other games, artificial intelligence systems beat even the best humans at poker. Poker is a game of imperfect information (because some cards in play are concealed) thus making it harder for anyone (including a computer) to deduce the final outcome of the hand.
Secondly, the broad set of hidden information of the tiles makes Mahjong a much more difficult imperfect-information game than previously studied ones, such as Texas hold'em poker. Thirdly, the playing rule of Mahjong is much more complicated because of the various actions involved.