That's because those games are nonrandom, and your opponent can be expected to single out the best move.

Algorithms for games like backgammon and poker that have a random element, do pay attention to density of states.

(Oddly enough, so nowadays do the best known algorithms for Go, which surprised almost everyone in the field when this discovery was made. Intuitively, this can be seen as being because the game tree of Go is too large and complex for exhaustive search to work.)