Google turns up only bits and pieces to this question.
SAE, Society of Automotive Engineers was formed in the early 1990's to develop technical standards for automotive (and later on aerospace) industries. This would include fasteners and the tools used on them.
Fractional is a description based maybe on the English measurement standard (?) that was adopted and is self explanatory.
Really, fractional (vs. metric) is a better descriptor than SAE. Over the years SAE must have taken over as the description of the "American standard" vs. metric.
The only analogies I can think of are bad: Kleenex tissue became just "Kleenex" for anything used to blow your nose?
Besides tools and fasteners, other things like drill bits are also described as fractional.