The French messed it up in the 1800's. When setting up the metric system, they had to come up with a distance for the meter. Their approach? Take a measurement of the longitude of the earth and divide by 10,000.
This single, idiotic, lazy-ass decision made life hellish for people around the world. How? For units of weight, volume, or even long distances, it really doesn't matter if a kilogram is 2.2 pounds or 2.3 pounds. It also doesn't matter if Pepsi comes in 2 litre bottles or 1.9 liter bottles. It also doesn't matter if you buy fuel by the litre or by the gallon. You buy all this stuff essentially in bulk, and the quantities are arbitrary.
What does matter is small distances, specifically for machine tools. The French made the inch equal to 25.4 mm. If instead they made the inch 24.0 mm or 25 mm, then everything would be okay today. But with 25.4 mm, you end up with 1/4-screws and M6 nut. They look the same to the naked eye but they don't fit together and need different wrenches to drive them. This is where distance is critical; for small distances, you have a lot of fixtures and hardware out there which is designed to match specific distances at close tolerances. Since the metric and SAE units are off by just a little bit, you end up with a world where everyone needs duplicate sets of tools and stuff that's incompatible with each other because they were fabricated to different units.
Sure, when everyone is on metric the whole problem goes away, but the laissez-faire, overromanticized decision of the French will continue to be a pain in the neck for years to come.