There's a few reasons for this. First, the 10-based units of the metric system isn't in fact its biggest utility--it's the fact that everyone agrees on the siz

There's a few reasons for this. First, the 10-based units of the metric system isn't in fact its biggest utility--it's the fact that everyone agrees on the size of the meter, as opposed to the traditional foot, where every country had its own slightly different length of a foot. With those other customary units pretty thoroughly dead, there's little confusion as to the size of a foot. Second, as a few people have noted, customary units actually tend to be somewhat more convenient to use than a strict metric system. Note that the metric system comes up with 1 liter = (1/10 m)^3 or 1 hectare = (100 m)^2 to try to synthesize useful volume or area units from regular length. In many regards, Fahrenheit is really a more comfortable measure of ambient air temperature than Celsius is. The metric system's fastidiousness in 10-based increments is more useful if you're doing tasks like measuring the area of an irregularly-shaped plot of land, or scientific calculations, but is less useful for more pedestrian tasks like measuring out how much flour you need for the cake you're baking. Third, it takes a very long time (at least a generation) for people to get used to new units. You see this in the UK, where imperial units are still used for lots of things despite official metrication. Residual use of customary units for things like cookbooks or tool dimensions will take a very, very long time to root out. Finally, there are lots of things that depend on the precise units that are hard to move. The US actually has two definitions of length, the international foot (which has 1 in = 2.54 cm exactly), and the survey foot (which has 39.37 in = 1m exactly). It's been trying to phase out the survey foot for decades, and that's still in progress because quite literally the coordinate planes that are used in several states for plotting land in the US are based on the survey foot. Even small differences in units there can lead to inaccuracies of several feet in determining the bounds of your plot of land.