Because the concept of the "light fighter" doesn't make much sense.
You have to use a full size cockpit so there is no savings in material or volume (unless you can recruit horse racing jockeys to be be pilots) in the cockpit area.
You have put a full suite of instruments in the cockpit so there is no cost savings there.
You should put in a standard radio so there is no cost or weight savings there.
You seem to be desperate to prove that the very concept of a "light fighter" was inherently impossible, and yet...
Now we can shift to things like not putting in the normal amount of guns. This keeps the weight down but it means you need more planes to get the same number of guns into the combat area, so the cost per gun is not much different.
I really don't buy that chain of logic. Hurricane Mk IIC had twice as many guns and probably ten times as much firepower (you can feel free to do the math) as a Ki-43. and yet, in Burma and India, over and over and over across at least two years, the Ki-43 proved vastly superior as a fighter, in the following ways:
Ki-43 pilots survived combat encounters more often.
Ki-43 pilots shot down many more Hurricanes than they themselves lost to Hurricanes
Bombers protected by Ki-43 pilots survived their missions at a higher rate
Enemy bombers protected by Hurricanes were lost at a higher rate
And it also had much better range and endurance.
The idea that victory comes down to a particular weight of ordinance brought into the battle area is taking the logistics side of the argument to absurd lengths. It doesn't matter how many guns you are carrying if the other plane gets the one little itty bitty bullet into the cockpit or your radiator. And that seemed to happen a lot with a lot of these "lighter" planes.
And that slops over into the engines/propellers. If we need 30% more 700hp engines to get the same number of guns into the air as we need 1000hp engines what are we saving?
See above ... and I guess the converse of this, playing devils advocate:
Do we need
two 1,600 hp engines to get five guns into the Theater?
Do we need 6,000 kg of aluminum and steel and oil and fuel to accomplish the fighter mission? Or will 2,000 kg actually get the job done just as well? In part, this depends on what the fighter's mission actually is, which also depends on the Theater. But here's a hint: There was more to the war than the Battle of Britain and the 8th Air Force Strategic bombing campaign.
Do we even need five guns? Or eight? I think in part that depends on what we need to shoot down. But clearly some planes were doing pretty damn well with just two or three guns.
And this is from a 1939-40 point of view. Once you start putting in armor/BP glass and fuel tank protection things look even worse for the light fighter. You need to protect the pilot and fuel to a certain level, using thin armor or BP glass because you used a cheap/low powered engine means you loose more pilots/aircraft and the cost of the substandard protection is wasted along with the aircraft.
As Tomo was articulating, which I thought was obvious - This was an arbitrary category based on an observation of the wartime facts. You seem to want to imply that there is only one way to saddle this particular horse. And yet, we can see viable wartime fighters ranging in gross weight
by as much as 8,000 lbs.
Yes things did change over time. Maybe at some point, and for some missions, one P-47 was worth two Ki-43s, but I think that took quite a while. In 1943 I don't think you can actually make that argument.
You seem to have an anti-American bias.
I really don't, I'm just making a point.
The US did do some not too bright things at times during the war. The insistence of using six .50 cal machine guns as a standard armament was probably one of them. This seems to have been down with little or no actual combat experience to justify it which did lead to some of the size/weight problems of the American fighters.
The thing with the guns probably could have been cured by adapting the
motorkanone approach. They figured it out for the P-39 so the Americans could do it, but they just chose not to. Probably because they couldn't figure out how to make the 20mm cannon work. I don't know if that was just a lack of sufficient emphasis in time for the war or what.
There was a kind of "Gigantism" to a lot of American planes during the war, and it went way beyond the guns. I think the issue was more on the planning / spec side than the firms involved, because some firms made planes that ended up being purchased by other European countries and they were comparatively thin and light, but when the same firm made planes to US specs they were much bigger and heavier and carried more than twice as many crew. And it's highly debatable that they were any more effective.
But I don't think you have to break it down by nation. By
time does make sense. There is an average fighter weight for 1938, 1940, 1942, 1944 etc. And yes it did gradually go up, at least to a point.
And there were some fighter planes which were maybe 80% of the size and weight of the average, but seemed to get the job done pretty well. I was calling these "Lighter fighters"
And there were some fighter planes which were more like 60% of the size and weight of the average, and I'd call those 'Light Fighters'.
That is the definition part. It has nothing to do with a particular program. The US tended to make fighters 120% to 150% as big as average, probably (guessing). Some of these worked out to be pretty good anyway, but it did take a while, and it's probably a luxury associated with having basically unlimited fuel and raw materials.
Ultimately, I think it's debatable whether this approach was better or worse. Whether lighter, average, or heavier fighters were better, on balance. Whether "light" (as in, 60-70% average size and weight) was even viable is another matter, but I wouldn't be so quick to dismiss it or so glib in doing so.