And that measurement they got was ultimately off, which is why the meter became (until its later redefinition), for all intents and purposes, "the length of some arbitrary metal bar".
Clearly, "some fragment of roughly how large the Earth is" is a base unit designed first and foremost for human-scale measurements, and definitely not by self-fellating academics that would never seriously have to use the thing (and whose work does not, by and large, depend on the base unit used; Americans use thousandths-of-an-inch for everything under 1/64th). Fortunately, metric time was too weird even for the French.
>but by measurements everyone on the planet could reproduce using the sun and a bit of water, at least to a very acceptable approximation.
By contrast, the "foot", which is literally named after the device used to approximate it. The median human being has two of these. In fairness, you usually have to be taught which part of one's body approximates the inch and is thus harder to do.
>join the metric system
The metric system was initially imposed at gunpoint by the French upon most European nations in the first place due to the French going batshit insane just before 1800 and conquering most of the Continent. The Russians, being a nation that rather famously was not conquered by the French during that time, would retain Arshins until Communism (at which point the Soviets would impose the metric system, also at gunpoint- kind of sensing a theme here); the English would retain the Imperial system even longer than that.
It's also why people living in the only nation that really has a choice all measure distance and height in (square) feet, inches, and kilometers (per hour), human weight in pounds (and very rarely ounces), most other weight in grams and kilograms, non-cooking-related volume in liters, and non-cooking-related temperature in celsius.
It would have been objectively better from a usability to have just set '1 meter = 1 foot' and call it a day- could have even been (one of) the French foot, too. But no, just had to fuck it up for everyone who actually has to use it because "we know better".