Getting 5 kills was really hard! Remember that on each side there were thousands of aircraft produced, along with pilots to fly them. With that in mind 300 to 400 aces really isn't that many. The fact of the matter is that often in WW1 combat a shootdown meant the pilot was dead - there was no cockpit armor or parachutes to save a pilot. For every kill by a Richtofen or Albert Ball you had a fresh, green pilot with less than a week of flight time who was either KIA or forced to land with a dead engine - probably shot down by an ace accumulating kills for his record.

These new pilots, depending on the time of war of course, had extremely low life expectancy. A new British pilot during Bloody April had one of less than a week! This is assuming they even made it through training, aviation was at its early point and accidents occurred at a much higher rate than would be seen in WW2 or later periods.