Originally, units of measure were people-centric: a cubit was about the length of someone’s forearm; a stone was how much a big stone weighed; a grain, how much a grain of cereal weighed; a day lasted from sunrise to sunrise, or sunset to sunset. These were units that, while making little logical sense otherwise, people could easily relate to.
But at some point during the Enlightenment, scientists realized that they needed units that would make their jobs easier, and so they made ones that lent themselves to fast calculations: the meter was supposed to be 1/10,000,000 of the Northern Hemisphere’s circumference, and all the related units were multiples of 10. The kilogram is based on the weight of a fraction of a cubic meter of water. Our days now begin at midnight because astronomers calculate days using the more accurate sidereal time, which needs to be observed at night, since it’s based on the position of stars. And of course, the temperature measurement above: Celsius is based on the freezing and boiling points of water, since water is such a crucial substance to chemists.
Scientific measurement systems are great for scientific purposes, but the length of a meter or the weight of a kilogram means nothing to the layman, a day that starts at midnight makes no practical sense, and outside of cooking, no one cares at what temperature water boils, but everyone wants to know how many layers of clothing to put on. Fortunately, thanks to American exceptionalism, we still use practical units for a lot of things: the foot is still about the length of a man’s foot, an inch, about the width of a thumb, and a cup is still the size of a cup.
The Fahrenheit scale is interesting because it was scientifically developed, but it was centered around people: 0° was the coldest substance that could be concocted in the 18th century, 32° was ice water, and 96° was body temperature. And so it was 32° between really cold and cold, another 32 between cold and comfortable, and a last 32 between comfortable and really hot. It later got a little bit of the Celsius treatment, and the scale was adjusted so that water boils at 212°, which moved body temperature up to the current, awkward, 98.6°F. But, for most purposes, Fahrenheit, and the rest of the American system, is ultimately just more practical for everyday use. The metric system is great for science and engineering, but, aside from having to remember how many cups are in a pint, there’s no reason the other 90% of the population should know how much a liter is.