On some/many turnarounds these days, there are no engineers, so the pilots must perform a proper walkaround check. Once airborne, we can't pull over to get out and check what's causing the funny noise.
Meanwhile the loose cowl might have just severed the fuel line or damaged the FADEC, so now you are down to one engine, or possibly none. Pretty stupid really, since a two second check of each cowling would have prevented all that from happening
