Except there is a defined way of determining a gram in weight. It's the weight of one cubic centimeter of pure water. You are correct in that it's something the rest of us just said, "Sure." and went from there.
Why I can't actually be wrong in my opinion when it comes to WAR, is it's an opinion, just like the values assigned in war. If the person who designed the formulas had different opinions on the various parts, then the pieces put into it would have a different value giving a different results. A bias, even when applied evenly, is still a bias.
My point being, you tell 10 people who have no concept of what baseball is to come up with a formula to represent the percentage rate a which a person comes to the plate at gets safely on base with no out being made through their own action and they come up with what we have already for OBP. Tell those same people to determine what any one player's value over a readily available replacement would be and you'll get 10 formulas.
My real point is this. You tell me a player was a 3.5 WAR, you may have told me he's a good player, but you haven't told me what makes him good. You tell me that same guy has a slash line .290/.360/.480 and a RISP w/ 2 Outs line of .320/.410/.560 and I know I got a guy who's a solid bat, good at getting on base and delivers fairly well when an inning is tight. But according to WAR, Kolten Wong was worth the same as Joey Votto.