Ever Heard the Phrase “Win the batttle but lose the War”?
Doesn’t matter how many little skirmishes the horde might win, the Alliance still win the big, drawn-out battles…
Also, Most BGs are like Time Walking PvP fights that just have players repeating previously fought battles, battles that canonically have already been resolved (usually in the Alliance’s Favor.)
Also, if player actions in game meant anything to the story, ALL the Leaders of both factions would be dead for 15 years considering how often they were killed by player raids.