I seem to be completely missing this. Ships have defensive ratings and upgrades and such, can angle deflector shields to shift defenses around, and get to choose which zone any incoming attack hits. Why wouldn't a pilot just shift everything to a single zone and always choose that zone as the target? Does defense decrease at any point, like shield losing power, or could a pilot have something like five setback dice to every incoming attack forever, even when the ship is incapacitated? Can't seem to find a rule on this.

Listen to episode 15 of the Order 66 podcast, it is explained fairly well.

As to decreases...check out the critical hits for vehicles.

The "get to choose" point is I believe where the confusion comes in. If an opposing pilot succeeds at the "gain the advantage" action then they would get tot determine where their attacks land, which would be in the shieldless zone I believe. If the PC's are in an stock YT-1300 that's a Daunting (4 difficulty) roll against TIE Fighters (traveling at top speed) but only a Simple (1 difficulty) check for the TIE's.

Also I would have to check, but their may be some Talents that remove setback dice from gunnery checks. Not sure.