Largely because they couldn't. While air force commanders did claim they could bomb the Germans and Japanese into surrender, they never quite managed to do so. Even in Vietnam, which dropped far far more munitions, the air power didn't win the war.

Part of this is simple reasoning. Bombs can cause a lot of damage, significantly so when you drop them in large quanities, but they don't really force someone with a rifle to stop unless they hit that person. They also had other major limitations that meant a dedicated force could usually not be bombed into surrendering. Limitations like you can't really fly constant bombing runs over everything, non guided bombing (dumb bombs) aren't very accurate, a bombing run costs supplies even if nobody gets damaged, and such.

Strategic bombing, especially in WW2 Germany, was no where near accurate enough to kill every soldier, and USAAF 8th air force wasn't usually aimed at soldiers anyhow (they preferred manufacturing and production, or cities). Adding to this, Germany was very good at repairing, replacing or simply rendering difficult to bomb the facilities that would be prime targets.

There are some real signs that strategic bombing simply wasn't cutting it. By 1945, despite constant bombings by the USAAF and RAF bomber command, the German Industry was far from crippled and at times actually exceeded its original abilities. That isn't to say that the bombings didnt have an effect, Doritz admitted it causes headaches, just wasn't war ending. Another factor however is that even the USAAF and RAF realized strategic bombing wasn't

Perhaps however the most damning reason why strategic bombing was not sufficient is that while the US and UK could spend forever and a half running bombing runs on the Germans, the Soviet army was far less safely secured. Where the British and US had the luxury of the English Channel, and a significant fleet, the Soviet army had a direct combat line with the Germans, and opening up additional fronts mean relaxing the struggle on the Red army and its allies. Remember that the Eastern front saw the biggest share of the fighting even after Normandy. Freeing up France was also probably a "To do" for Allied brass. Not to mention ending the war before war weariness caused trouble. England had been fighting for 6 years by the time the surrender came after all, prolonging it was not helping.

For an idea of how inaccurate the British were, check this graph out. Note that at the end of the war (where there is a sharp increase) the RAF under Harris were doing the equivalent of "not aiming" and just carpet bombing huge areas. When you stop trying to hit something specific, your accuracy goes up.