"Combinatorics, probably. In the case of Go, anyway (I have no idea how Bridge works). Although there's only one kind of playing piece in Go compared to six in chess, each player gets 180 of them, potentially all of which can be used, on a board with 361 (=19 x 19) positions, compared to just 64 squares on a chessboard. After just a couple of turns, the number of possible games becomes astronomical, much faster even than chess. Also consider that the number of distinct board states decreases as a game of chess progresses, as pieces are taken, while in Go the players add pieces to the board."

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