The short answer

Pirates are a big reason why the United States never started using the metric system when other countries began using it. And now the U.S. doesn’t use the metric system because of the immense upfront cost of switching and a feeling of patriotism for the current system.

The long answer

This question is best answered in two parts: why didn't the U.S. use the metric system to begin with, and why hasn’t the U.S. switched to the metric system?

Why didn't the United States use the metric system to begin with?

Pirates. Literally pirates are a big reason why the U.S. never started using the metric system. I'll explain.

The metric system was invented by the French in order to standardize measures, which often differed not only from country to country but from town to town. After many attempts, the new system was finally put into place after the French Revolution. Liberté, égalité, fraternité (and standardization)!

Around the same time, the brand new United States was also dealing with the flaws of their inherited British system of measurements. Thomas Jefferson, a fan of French scientific ideas, lobbied for the U.S. to implement the new system and sent for the French scientist Joseph Dombey to convince others to switch to a more logical system of measurements.

But there was one problem: Joseph Dombey's ship had blown off course and been captured by pirates. He was held prisoner in the Caribbean until he died. Back in United States, people were already used to the British system and so they stuck with ounces and pounds, inches and feet.

Why hasn’t the U.S. switched to the metric system?

To start with, the U.S. actually does (kind of) use the metric system: Americans drink 2-liter soda bottles, run 5Ks, and take 100mg of aspirin. Why is that? In 1975, Congress passed the Metric Conversion Act, which declared metric as the preferred system of the United States, but President Ford emphasized that conversion was "completely voluntary." Some companies, like Coca Cola and Pepsi, figured it was a matter of time before all products would be sold by the metric measurements so they sold 2-liter bottles instead of half gallon bottles.

So the U.S. has a patchwork of different measurements – why hasn't it made the switch to purely using the metric system?

Reason #1: It would cost a lot of money and effort for the U.S. to switch to the metric system.

If we were to make the switch to metric purity, there would have to be a huge upfront investment. Textbooks would need to be reprinted, road signs replaced, etc. The private sector would also need to spend a great deal of money on the switch, and it's easy to predict that there would be intense lobbying to stick with the system we have. In fact in the 1870s, industrialists who had already stocked their factories with non-metric system equipment used their influence to thwart an effort to make the switch to the metric system.

Reason #2: Some people feel that the metric system is “anti-American.”

A sizable portion of Americans have an affinity for the system because of a feeling of patriotism. Anti-metric sentiment stems from a desire for America to stand apart from the rest of the world, perhaps as a display of pride and nostalgia for the current system.

Shortly after the Metric Conversion Act was passed, Senator Chuck Grassley declared, "Forcing the American people to convert to the metric system goes against our democratic principles." While the rest of the world may view the U.S. clinging to its current system as penny wise and pound foolish, America has decided that its streak of individualism extends to the yardstick.

Sources

Basu, M. (2015, July). Refusing to give an inch America's only Metric Road. CNN. Retrieved December 13, 2022, from https://edition.cnn.com/interactive/2015/07/us/metric-road-american-story/

Chandler, N. (2020, March 9). Why do Americans sell soda by the liter but milk by the gallon? HowStuffWorks. Retrieved December 13, 2022, from https://money.howstuffworks.com/why-sell-soda-liter-milk-gallon.htm

Lerner, C. (2021, July 21). Why the United States doesn't use the metric system. Reader's Digest. Retrieved December 13, 2022, from https://www.rd.com/article/why-u-s-doesnt-use-metric-system/

Martin, K. (2017, September 19). Pirates of the Caribbean (Metric edition). Taking Measure. Retrieved December 12, 2022, from https://www.nist.gov/blogs/taking-measure/pirates-caribbean-metric-edition

Plackett, B. (2020, August 15). Why doesn't the US use the metric system? LiveScience. Retrieved December 12, 2022, from https://www.livescience.com/why-usa-not-metric.html