Well, if you take that to its extreme you would only have one resource (and I'd find that a bit boring, but there are certainly other games just using one or two resources). Or on the other hand you could probably argue that there are reasons to split the existing resources up in a ton of different ones. But for 0 A.D. it was chosen to have wood, food, metal (which includes both currency and raw material), and stone, and at this point I really don't see why we should change that (especially since there are always going to be people who want fewer/more resources and arguments as to why). Four is a middle ground between having so few resources that there's no variety and so many that it's impossible to balance the game (or for the players to keep track of them for that matter).