One problem with units is defining them precisely.
For example, a meter is ostensibly “one ten-millionth of the distance from the equator to the North Pole along a great circle”. That’s not exactly precisely defined because the earth isn’t a perfect sphere.
So currently, a meter is defined to be the distance light travels in a vacuum in 9192631770 / 299,792,458 hyperfine structure transitions of caesium-133.
Rather than doing the same sort of thing with updating the standard definition of a foot or pound, the US just piggybacked off the work precisely defining metric units and defined imperial in terms of metric.
So now a foot is officially the precise distance light travels in some number of hyperfine structure transitions of caesium-133, and the US government didn’t need to do a thing.