It's chronic, structural or systemic, I'm tempted to say. DMing has generally been either hard in the sense of requiring talent/creativity/energy at a high level, leading to burnout (an issue, IMX, with 0e/AD&D& 5e, particularly) or hard in the technical and time-sink sense (an issue with 3.x/PF and many other systems) also leading to burnout, or at least frustration. The player side may be challenging or even frustrating in play, but has only been a time-sink/technical slog in the WotC era (and is less so in 5e). So we have way more players than DMs because the player side is easier and players don't burn out.