"Why" questions need to be sent to the devs. All we can to is guess, observe, and play the game the best we can based on our experience with it. It's a little like real world battles and war. Those who fought it best did so by trial by fire and listening to those others who survived.
The status bars actually work better than it would seem (although you get the nice mouseover.) Very small differences between similar crew (10% or less) don't produce strongly observable and reliable benefit when in the heat of battle. Did the gunner on the AA gun shoot down that last Betty because he had 80% instead of 70% skill, or did he get lucky, or did the Betty pilot get sloppy? Do the best with the crew you can and it'll pay off in the long run. The system is complex enough that a fool proof system is hardly worth the trouble.
-Pv-
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