It's like any human skill... calculate as much as you can (or as much as is useful) and then use your knowledge to make your best educated guess.
As for why not calculate more, it just depends on the position. Imagine asking "why did you centralize your queen in the queen and pawn ending? If you had calculated 50 moves you'd see it didn't quite work." Well sure, but that's not practical. When there are a million variations you play based on principals.
Don't they say that Go is even less about calculation than chess? It's weird because to me it seems like it would be the opposite, that it's like, you're either going to get the territory or you won't, and you need to look far ahead to get any sense of it. It seems like it would be more mechanical and counting based.