Multinational forces would make the Allies win too easily, such as happened in history. We want a board game that every side can win.
Making this game more historical would make it boring. To be accurate you have to give US about 55-60 IPC on the pacific side and about 50 IPC on the atlantic side. The Allies will always win, such as happened in reality. But would you like that kind of game ?
Even more: you would have to customize all units: German Tanks should cost 6 IPC but attack at 4. US and UK tanks cost just 4 IPC but attack and defend only at a 2. German infantry should cost 4 IPC but attack at 2 ,3 with artillery, while Russian infantry just cost 1 IPC and is an 1/1/1 unit (see battle of Stalingrad where Russian soldiers were sacrificed as cannon fodder). German fighters had just 3 movement while UK and US fighters had 4-5 range. Rules would become even more complicated, and believe me it is already complicated ! You can’t start a game with beginners without about 30 minutes of rule explaining and at this forum the rule clarify section is the most visited one with the most replies…
Despite all that, you are right, but it’s not a flaw, it was planned this way to balance the game.