The losses due to breakdown for fighters are related to fighters' reliability. You cannot mix air combat losses with breakdown losses. For instance think about land combat: for consistency you should mix breakdowns with battle loss. Instead, in game, the losses are kept separate by cause/nature.
Why not? - Reliability affects attrition rate - which IRL (with aircraft) includes lossed due to crashes. If plane is hard to control for a new pilots, leading to higher crash chances, it could be represented as lower Reliability value.