Going to the metric system require us to exchange a system with a several non-decimal conversions (i.e. 12 inches = 1 foot, 2 cups = 1 pint, 16 ounces = 1 pound) to one with decimal conversions, of which 16 are required to be memorized in the order of increasing or decreasing units.
Most people never need to bother beyond milli, centi, desi and kilo. And, hey, the conversion is always decimal based so you just need to know the names. Desi and centi are not "regular" 3 orders of magnitude units but you do need to measure 1dl for cooking and 1cm is a lot more useful measure than 1mm for everyday business.
Beyond that, doing actual calculations, even simple ones, gets ugly fast on silly units. Figuring out the cubic kilometer thing is public school math, many people would get it wrong for sure but the principle is exactly the same never mind which units are used. Now most people don't do math with real life objects on day to day business which explains why so many people don't "get" metrics.
As I said, foot and pound are perfectly good units. Just adjust the adjacent stuff to work on decimal scale and you're peachy.