There is a belief that wrench size is the across the flat distance of a bolt head/nut. any reason other than this for designing wrenches in these standard sizes?
Spanners and all hand tools are based on the bolt head size, eg, 3/8" across the top of the flats of the bolt head, note, this is not the bolt shank size, the threaded part. Because most widespread use of tools began in the US, the Imperial system of tool sizing came from there. In the last 30 or so years, the metric system has become widespread, but as aviation is largely based on the US system, the imperial system of measurement largely remains. There were some other forms of tool measurements uses, the UK BAE and Whitworth sizes but fortunately, these never became widespread, otherwise, your toolbox became a nightmare of tool sizes. Yes, been there, had that.