A cowling falling off probably isn’t a manufacturing defect. Someone forgot to close the latch, and the pilot failed to spot it on their walkaround before takeoff.
I wonder.. how hard would it have been to add a switch to those latches (maybe not all of them, but at least the ones most commonly opened during routine maintenance), so the plane’s computers know what state it is in? The plane could generate a warning alarm in the cockpit if you start the engine without the cowling being latched.
It’s also not a Boeing problem, they don’t make the engine.