That doesn't (necessarily) mean a proof can't be found that a certain ruleset leads to optimal play. Tic-tac-toe can be solved without examing the entire state space of the game.
That's mostly because of trivial symmetry and even taking that into account Go space is in the most literal sense astronomic, simple numbers don't work in describing just how large it is.
And it is very likely that it will never be, the number of combinations is simply too large.