I need to be very precise here, but it is not right to say that we know what the best move is in any chess board position. What we know is what move a chess engine would make that would win the game against a human, or sometimes another chess engine. We know a winning move; not the best move.
Winning in chess is not the same as knowing what the best move is. Having chess engines that can beat any human in chess is not the same as knowing how every possible game will develop. The latter is known as "solving chess" and we don't yet have that. We have fully solved games like tic-tac-toe, backgammon and checkers, so that there exist databases of every possible path through those games but this is not done for chess, and there is no hint that it is even possible to do given that chess is such a combinatorially more complex game than those.
More to the point, the upshot of the fact that we now have super-human chess engines that can beat any human in chess (and Go and shoggi) is that there is no longer the will to research the bigger question about solving chess. The solution of the minor problem, how to beat humans in chess, has sidelined and displaced research in the major problem, how to solve chess, and we will now never find the solution to the latter.
And if that reminds you of something, well, yes, exactly.