That's because pounds use force as a base unit, mass is kind of incorporated into it the notion of a pound, whereas the metric system generally handles the unit

That's because pounds use force as a base unit, mass is kind of incorporated into it the notion of a pound, whereas the metric system generally handles the units for force and mass separately. The imperial system actually does have a rather obscure unit called a slug which is equivalent to the kilogram (in terms of being purely a mass quantity) in the metric system, but since the pound already incorporates that term ( bundled together with the gravitational acceleration factor) it seldom sees action.