As I recall, the sealed beam was mandated primarily to control the beam throw pattern and the overall size of the lamp.
The standardization prevented people from using huge search lights out front, blinding oncoming drivers. (There were no cell phones back them to avert your eyes from oncoming head lamps at the time.)
There was also the need to keep left hand drive headlights from being operated here in the right hand drive states.
Further, the reflector shape and the glass mold pattern made it much more likely not blind oncoming drivers. Citations due to modifications, were easy to keep independents "in line". In short it was far easier to mandate and maintain a fleet of vehicles where the headlamp design was fixed and equalized among the populace, without a detailed and complicated codification of field test parameters.
Yearly or routine auto inspection weren't performed and the populace wouldn't have stood for it anyway. The prohibition era had taught the populace to circumvent government controls. If the US only allowed one type of bulb into the US, "creative alterations" became fewer and farther in between.
It also prevented the replacement of jeweled or focused cover glass with clear, thus blinding all oncoming drivers.
Once a law goes into the books, it tends to stay there, if for no other reason than to maintain the status quo.
You may understand that in 1940, there was a lot of sparsely rural areas, wide open spaces, and farmland. People were fiercely independent and had learned to make do with very little (depression era and WWI rationing). Anyone that has seen some farm machinery repairs, knows that elegance or design correctness is largely ignored in favor of getting the planting or harvesting done.
If you lived your entire life in cities or densely populated areas, where police are more prevalent, there was/is much less pronounced "offensive equipment" population, imo.
That's my recollection, anyway. Besides, we Americans didn't give cr@p what the French, British, Germans or Dutch thought about our headlight legislation in the 30s and 40s.
