Fahrenheit today is literally defined through Celsius

The same as pretty much every unit they use

At this point, that’s basically every unit other than the seven fundamental units. Degrees Celsius is defined from the fundamental unit Kelvin.

Plus the actual definitions of those fundamental units were defined based on historical measurements tied to former definitions. Today the second is defined around the frequency of the cesium-133 atom, but it was traditionally measured as 1/(60 x 60 x 24) of the time of a single rotation of the earth, which stopped serving us when we realized the rotations had too much variation between days. The meter is currently defined around the speed of light and the second, but was previously defined in terms of what they thought the Earth’s circumference was, and then a metal bar they kept in Paris, then based on the wavelength of light emitted from a transition in krypton-86. Same with the kilogram, currently kept at Planck’s constant but previously based on a particular chunk of metal that was mysteriously losing mass over time, and before that defined from the density of 4°C water and the definition of the meter.

Conventions are important. The history of how we got to particular conventions can often be messy.