The base units are---or were in the first iteration---derived from particular relationships to real-world measurements, but they're abstracted: nobody needs, in a practical sense, to have the earth's circumference be an even number of units, or a unit of mass that equals the weight of water of such and such a volume at sea level. It wasn't as if the people who invented it assigned names to a standardized version of units already in use: they intentionally departed from that kind of system.

Which makes sense, given that the goal was to achieve scalability through decimalization. And that, in its way, is a sensible approach to a system. But it's sterile and inhuman in many ways, too.

A pints a pound the world around is no more meaningful tan a milliliter of water equals a gram.
Memorizing conversions it's time better spent on basic computation, or reading or even doodling.

Rods and chains weren't arbitrary, they were derived from actual tools used by surveyors to measure property. They were the opposite of arbitrary; they were organic.

What they were was cumbersome, which was a different, though real, problem.

Those lengths were the very definition of arbitrary.

The French Imperial system's foot was actually 1.066 ft. long.