It's a bit like dealing with miles, furlongs, yards, feet and inches even though a decimal system would obviously be better? We manage to cope in the UK and the US ...
A decimal system is not obviously better. There is nothing inherently more logical and rational about decimals. Decimals aren't even necessarily easier to work with. Try dividing a decimal measurement into thirds or sevenths.
In fact, most of the old 'trade' measurements had more thought put into them. They weren't random. They were designed to be useful in everyday circumstances a tradesman would encounter. Having 12 inches in a foot, for example, meant that a carpenter could easily divide a measure by 1/2, 1/3, or 1/4. The fluid measurements were even more ingenious, with the same measurement being able to be easily divided in halves, thirds, quarters, fifths, sixths, or sevenths. For example, the tun was a barrel 42" high and 42" deep (notice again, the use multiples of small integers), that when filled weighed one tonne and contained 252 gallons - again divisible by small integers. And each fraction of something had a name, which is far more useful in a system where a large portion of your buyers don't have an education, and may be illiterate or innumerate.
What is called the "Imperial System" evolved to meet real needs. The metric system on the other hand was devised primarily as a political statement, with very little regard to how it would be employed. Originally they even tried to divide time into decimal units, rather than the old 24 hours to a day, 60 minutes to an hour (again, notice 60 is divisible by 2, 3, 4, 5, and 6), and 60 seconds to a minute. But even the French would have nothing of that stupidity.
Metric resolves some important issues, but divisions of 10 is not inherently more rational than divisions of 12 or some other number.
