I think they're just not well understood by most people. 1000Kg is a small car. 1Mg is...who the fuck knows. You can tell someone a building weighs 1Gg but they're gonna stare at you funny until you tell them it's about 1000 small cars.

OTOH, 1KHz has no basis in physical reality for normal people but they know 1KHz is 1000x faster than 1MHz so it works nicely for things like "is my new computer faster than my old computer" (at least a decade ago, I don't see clocks advertised much at all anymore).