The Metric system is a top-down system vs. the SAE system which is a bottom-up system.
Metric was conceived during the French revolution to replace the Avoirdupois system, which was simply a continental version of what we have in the SAE/USS system, or Imperial system. It was designed by a committee, and even then it is not internally consistent. Take for example the meter, which is based on a measurement of one-quarter of the world. Why not base it on the full circumference? Well, it wouldn't come close to being a useful measurement that approximated the yard. Another example, the gram. Which is simply an object, kept under lock and key that they decided was the basis of the measure of weight. It isn't based on anything except some dude, who was not a king, saying so. No more, no less. Often one hears claims that it is internally consistent, but the meter-liter-gram test shows how false that is; a cubic meter of water is much greater in volume than a liter and weighs much more than a gram at the same time. Now, there is nothing wrong with any of these points, but they simply show that any claims of internal consistency are false and that it is as consistent as Imperial measures.
As far as accuracy goes, either measurement system is only as accurate as the metrology used to measure it. In other words, as long as the consistency of the measurement remains standard, it doesn't matter what system you measure with. Feet, meters, tatami, arshin, lini, or chains, all are as accurate as of the measurer. And all of those systems are still to be found on the earth, modernity notwithstanding.
Further, the reason the United States is slower in changing to the metric system is pretty simple. Every. Single. Item. made before 1975 is in SAE. Everything. And to give you an example, that is every screw thread, every thread cutter, every machine tool, window frame, doorknob, and every other thing you can think of. And the cost of changing those items, all at the same time? There is no way to put a cost on this. And if you ask "all those other countries did it, why no us?" it is because every other industrial nation had adopted the metric system before the machine age, or they weren't an industrial country. Japan changed to metric in the mid sixties, Russia didn't become an industrial country until the Russian revolution, when it adopted the metric system. England, possibly the only exception, changed to SAE from Whitworth around 1960 at incredible expense and was only able to from the damage sustained in WWII. When they tried to change again to the metric system around the same time as the US in the late seventies, it killed much of the industrial might of that country. All the countries in South America? Canada? They were never industrialized.
And none of that is to say that Imperial measurements are better. They just are. It might not be perfectly accurate to pace along a piece of wood and say it's 10', but it will get you close enough. Or to place your thumb on an object and see if it is larger or smaller, giving you an idea of its size. And before the machine age, when every single thing was hand-made, there was little need for greater accuracy. Now? Well, SAE did take people to the moon, was used in machining airplanes, jet engines, ocean carriers, sewing machines, and every other thing you can think of.
