Um, there is a reference point. In the case of distance measurement, the reference is the Meter.
Bushel, peck, dram, furlong, stone, pinch, league... what? Too much memorization, and none of it makes sense.
That's like saying we have a length or weight reference point of the color blue to most Americans.
Until you have an automatic internal basis for your thoughts or comparisons it simply doesn't compute. Oh, you can pull out a tablet or a calculator and convert it to REAL NUMBERS, but only then do you realize how much, how large, or how heavy it is. Until that time you have to stop reading or tell the person to stop speaking to you while you do the math.
Meters are like yards - only not really- and it takes 3.8 liters per flush, wait I mean per gallon. (I learn a little metric every time I go to a public restroom). The issue is that until we have some sort of internalization it's just greek and doesn't mean anything.
I glaze over when you guys start quoting mm in here. I simply don't have a clue how big a mm is. Oh, I've read it before, but it doesn't compute internally or quickly. If it's a measurement that I really want to understand I stop reading and pull up my converter on the computer and figure it out.
There's also the fear that if we change over to metric the manufacturers will use it as a way to stick it to us. When Coke changed over to liter and 2 liter bottles the cost per ounce went up cause they figured no one would notice or be able to tell.