Its kinda really QNH/QNE. QNH is the local altimeter setting of an airfield, that you use up to 18,000 feet (in the US) that's called transition altitude. Above that everyone uses 29.92 (QNE), the barometric setting of a standard day at sea level (15 degrees C).

The transition altitude is a safe margin above any obstacles for that given country/continent. Its used so planes aren't constantly getting the altimeter setting of wherever they're flying over. Everyone has the same setting so it doesn't matter that its not accurate. You're not gonna hit anyone else, and its not wrong enough to be the reason you fly a perfectly good airplane into the ground.

Like you said, QFE isn't really used much. There's really no point in it when almost all modern planes will have a radio altimeter which work below 2500 feet, and that's what we use, not pressure instruments.