40k doesn't make sense???
Why a space marine can't take a cover save from a shoota because he has better protection from his armor than a sandbag barrier? (other than the obvious game balance issues that having stacking saves would cause)
Honestly the rules are an abstract and it would be incredibly fiddly to figure that stuff out. What if a unit of boyz has a killa kan blocking part of the unit and a battlewagon blocking the other. Which intervening model takes the hit? What about if your in cover and you have some grots in front of you? Did the cover block it or did the grots tank the shot with their face? Also what would happen if the intervening models were locked in combat? Too confusing to figure out and too many small dice rolls to resolve something that can easily be resolved with the current system.
Also it would be "the unit in front is hit" because you can't wound a vehicle and you might not wound the thing providing the cover.