The biggest factor was the scale of pilot training. The British Commonwealth Air Training Plan appears to have trained around 110,000 pilots and claimed to have produced a surplus that caused the program to start scaling back in 1944 (RAF Museum link below).
I can’t find exact figures on the American pilot training programs but I assume it dwarfed the British program. The only data point I can find now is that U.S. contracted flight schools produced about 250,000 pilots during the war, but I can’t tell what percentage of the total that was. In any case, it was very sizable.
I can’t find specific figures for the rates of Luftwaffe pilot training. Most sources say there was a shortage of pilots and fuel, but don’t give numbers. On the whole the Luftwaffe bureaucracy was not far-sighted nor very efficient and so training, tactics, R&D and strategy all lagged. It is well known that Germany lacked access to many war-essential materials on the volume necessary for a great war, which impacted the Luftwaffe in terms of production and fuel (Hart). If you read Hart’s excellent book you will find that for most of the war Germany was fighting from a position of scarcity.
Edit: In Hart's book, he states that around mid/late 1944, the Luftwaffe aircraft fuel request for full operation was 160,000 tons per month, asked for a minimum of 30,000 tons per month, and actually got 10,000 tons per month. Some anecdotes say this meant training was essentially stopped.
On the face of it, by 1943 the Allies were dominant in pilot training, aircraft production and had the logistics and resources to make use of them.
In terms of the balance in combat, the couple examples of aerial war of attrition in WW2 point to the trend that both the attacker and defender will be ground down, but the attacker will be ground down at a faster rate unless there is some other factor that is completely out of balance, like relative pilot skill or relative aircraft capabilities. The attacker must seek those key advantages while also relying on a larger scale of combat power in order to survive attrition and remain viable by the time the defender capitulates.
During the Battle of Britain in 1940, the attacker’s committed forces were larger than the defender’s committed forces, but both were grinding each other down at roughly an equal percentage (Price). In other words, both were on track to be depleted at about the same time, despite the quantitative disparity between the opposing forces. The attacker’s pilot losses are greater because they were losing more airplanes per day, and could not recover any pilots lost over the defender’s territory.
The same trends applied during the strategic air war in the West, although the scales became so lopsided in the favor of the Allies that they became dominant in the skies despite losing more pilots in action compared to the defender.