I know I’m only throwing more fuel on the fire here but it needs to be pointed out that the Horde’s immorality is front and centre to players. It’s quests, it’s cut scenes, it’s cinematics for the entire player base to see.
Any immorality the Alliance may have is mainly within novels & comics that only a fraction of the player base gets to see.
Most people just want to play the game and have no interest in the extras outside of it, therefore it’s understandable for many to see one faction as Autobots and the other as Decepticons.