American children are taught both the English and metric system. Industry uses both. The scientific community uses metric. Although it is not the "official" system, it is used. It missed being voted into use by Congress around the early 1800s/late 1700s by one vote. As for why it's not the official system, I'd only be guessing, but it seems as if we are slowly heading that way.
And because they use both some expensive mistakes have happened... remember the Mars orbiter/lander in 1999 that went into orbit too low? It was around a third of it's intended height... now what's the conversion factor between feet and metres? Do I hear 3.28? Connection? Yup. Computer software navigation components written by different teams were using different units. Oops.
What that has to do with the question escapes me. Whatever system was used, if it were consistently used, would have performed properly. What you are talking about has to do with the fault of the programmers, not the system.
We are taught both in school, but as in Britain today we just are used to the imperial measurements and don't want to change it! We even have a few extra that you don't seem to use - like "cups" for instance...