I think someone earlier is confusing "metric" w/"decimal" (aka "base 10"). Our currency isn't metric.
Until the Euro, English currency was all fubar'd, as well as many other currencies worldwide. Compared to some of them, US currency is down right simple.
Developmentally, we humans go through a phase where language and such is easily learned. Once past that stage, it is far more difficult to learn/retain language. Maybe it's the same w/learning some math concepts like the metric and "other".