It's a way to put a plot event in terms that an early 21st century moviegoer can quickly understand. Unfortunately, they sort of missed with that one... doesn't really stand up to even cursory examination.
You might even ask - why didn't they have automatic bomb arming devices? During WW2, bombs had automatic armers, wires that got pulled out when the bombs were dropped, releasing a small propeller that spun in the airflow and completed the arming after the bomb was well clear of the aircraft. Otherwise, a fragment from an exploding AA shell or bullet from an attacking fighter could hit the fuse and touch off the bomb while it was still in the aircraft.
Note that bombs contain high explosive, the 'high' indicating that a high amount of impact force is needed to set the explosive off. It's a safety measure, so you don't get a boom until you really want one. Even a bullet or shrapnel fragment won't set off high explosives, only a blasting cap or other device capable of creating a shock wave, can do that.
One exception in WW2: The detonators for the atomic bombs were not even installed until the bomber was well clear of Tinian island, the thought being that if something went wrong and the bomb blew when it went through the first stage of activation, they didn't want to take out their main bombing base.
One other exception: the kamikaze vehicles, both aircraft and the Kaiten manned torpedo, had manual arming methods, because the bomb never left the vehicle.
Presumably, the fleet, working with technology far in advance of ours, would have solved that issue for the same reason it was solved during WW2 - early arming means higher chance of premature detonation due to the enemy shooting at you and maybe hitting a bomb... as the fleet discovered, much to it's discomfort.
The alternative to that storyline would have been to come up with a plot that had the fleet obliterated for reasons that a moviegoer might struggle with... like maybe some sort of super weapon that a contemporary viewer might not understand.
An example of dated thinking evident in sci-fi: If you go back and view 2001: a space odyssey, you will find a few things that were clearly the result of a 1968 frame of reference: the video phone being a station and not a pocket phone, for example. That can be understood - even the concept of a cell phone didn't exist in 1968. (otherwise, that film was remarkably accurate, even with what we know today)
This one, though... a rare slip-up when they should have known better.