There's probably more to it than that. Humans do typically play Go on 19x19 boards, but Go is still relatively hard for computers (compared to Chess) even on sm

There's probably more to it than that. Humans do typically play Go on 19x19 boards, but Go is still relatively hard for computers (compared to Chess) even on smaller boards like 9x9, where the branching factor is reasonably comparable to Chess. And the branching factor in the kind of Go tactics problems collected as exercises in books for human players isn't that much larger than the branching factor in chess, either. Decades ago chess programmers could remark that their programs solved typical chess tactics problems faster than humans could turn the pages in the book. Even today solving typical Go tactics problems is somewhere between a very serious programming challenge and an open research question. Broadly speaking the difficulties in Go tactics seem to be similar to the difficulties that computers have historically had in Chess endgame tactics, which clearly wasn't a problem of branching factor: branching goes down in the Chess endgame. (However, today there is an important dissimilarity: the space of Chess endgames is small enough that modern computers can tabulate the solution for a significant fraction of the problem space beforehand.)