same reason a 2x4 is a 19x38 now...you are not going to change the dimensional size of something that will mess other stuff up. You change the size of a piece of lumber and you have to change all support brackets, plus plane down wood if doing a reno. You don't change a pop can because it would not work in pop machines and entire production lines....note that pop bottles came out after the metric system was common, so they are even metric numbers.
There are several forms of imperial measurement, varying from country to country. Many of the measurements are the same, but there are differences...why, because communication was difficult when they were developed, and nobody wanted to be the one to cave to another county's standard. I suppose it would kind of be like a position of power.
We used the British standard (imperial) because we were still a part of the British colony when imperial units were introduced/standardized a couple hundred years ago. The US was not, and elected to not follow this, likely to tout their independence.
Why teach dead math. It is touched on, but definitely not a focus. Metric is our standard, and has basically taken over. People under 40 really don't use imperial except for height and weight...and that is fading away. With imperial there are certain units (classes of measurement) that do not even exist, making some scientific calculations (more on the physics end) impossible w/o converting to metric and SI. SI units and calculations use no imperial units. imperial units are a pain to work with because there is not a standard denominator in general mat like metric (ie metric is in units of 10 for common math...basically until SI calculations are involved).
The imperial system is one based off of observation, the metric is one that was thought out and developed to be easier to work with.
A foot was the size of a kings foot at the time of development...standard made
an inch was the knuckle width of an emperor...12 of em in a foot...standard set
a yard was the length of his standard stride while walking through his court yard...standard made
a league was the arm spread of some sailor...standard set
this goes on and on, hence why conversions in imperial are a major pain. Yes, some logic was set later, but the base units were developed in a painstakingly way because there was simply no other readily available option at the time...at least the recognized the need for a standard.
Imperial has served its purpose...much like the horse and buggy, slate tablet, slide ruler, black and white tv, etc etc etc
now it is time to use a system that is easier to use. That is why it is not taught.