Because the way our motors are, they always, always, run better with the original equipment plugs.This is especially critical with the Skyactiv motors. Luckily our 5's have the MZR, so it's not quite as critical. But still, the price difference is only a few bucks each between the correct plug, and the "yeah, I guess it'll be okay" plug.
Just because RockAuto, AutoZone, or O'Reilly's says a part is compatible, doesn't mean it's the right one to make the engine perform as well as possible.
I can absolutely tell you this: if you ever have to replace any type of solenoid or sensor, ALWAYS insist on OEM.