If a game feels unfair then few players want to play it.
What players see happening in the game and how they interpret it can often be at odds with what the game maker knows goes on under the hood. Fairness is subjective: a game is only fair if its players believe it to be so. To the player it really doesn't matter how the game engine does what it does, whether it's actually balanced or full of hacks that break balance in their favour. They only know what they see and what they think they see. That can be very hard for the maker to understand.
Designing for fairness sounds like a simple principle, but because it's about feelings rather than facts it's actually very complicated. Particularly for multiplayer games.