Many, many people have wondered why the U.S still doesn't use the metric system for everything. It's decimal based and way easier to use. For instance, acceleration due to gravity in metric (like we use in physics) is 9.8m/s^2 which can easily be rounded to 10, an easy number to multiply mass by the find weight. However, in all my math and engineering classes, teachers always seem set on using U.S Customary Units, which makes gravity's acceleration be 32.174 ft/s^2 which is a ridiculous calculation to do in your head, and makes doing physics problems that much more time consuming. Plus, its super easy to convert units. Grams to kilograms you just move the decimal 3 places to the left. Simple.
And on certain products we use metric, other ones we don't. Why do we have a gallon of milk but 2 liters of soda? I decided to look this up. We got the U.S units from the British Imperial System (both are actually identitcal, but different words for some measurments). In the late 1700s, France was developing the metric system. Also at this time, the U.S and France didn't particularly like eachother too much so Americans couldn't travel to Paris and learn about the new system. In addition, it was decided that switching to the metric system could screw over a lot of people because it would be hard to make sure everybody has a centimeter that is actually a centimeter.
In 1866 America finally recognized the metric system saying,
"[it's] lawful throughout the United States of America to employ the weights and measures of the metric system in all contracts, dealings or court proceedings."
Although the metric system is recognized, it's not always used. In 1975 the Metric Conversion Act was passed which said everyone should convert to the metric system. A major reason that a lot of businesses and scientists and engineers have no converted to metric is because of the cost. If it were mandatory for every measurment to be converted to metric, the man-power and therefore dollar cost would be huge, particularly in complex blueprints like the ones for the space shuttle. Fortunately, some places have switched to metric in the U.S such as pharmacy, film, tools, and bicycles.
In conclusion, Americans are painfully slow with switching to the metric system. At least scientists seem to be smart enough to let us use metric, even if I still have to use silly Americaaaaa!!! "horsepower" in engineering.