Not that simple Paul. It isn't just rim dia that dictates tyre sizes but wheel width and stud size and pattern. Add to that aspect ratio, load and speed ratings, tread type and pattern and the variations increase, put the wrong tyres on and not only roadholding and handling but possible speedo errors could be the result.
My last 6 cars have been BMWs, Audis and now a Merc and certainly the BM wheels would not fit the Audi even though same rim size which in turn I doubt would fit my Merc, though I haven't tried the latter. My last BM had different size wheels on the rears anyway so you couldn't rotate them not that many people do these days. Annoying to have a perfectly good set of winter alloys and tyres only not to fit on your next car because the drum size and stud pattern aren't compatible.
As far as buyers being able to get tyres easily, that isn't always the case either. My car is recommended to have all season tyres fitted due to a "crabbing" characteristic on these models with standard tyres but despite the model range being introduced in 2015 there were no available tyres until 18 months later and even longer for the larger wheel sizes.
Wheels apart, the choice of tyres available is extensive and the car and commercial vehicle manufacturers don't just choose a size because that tyre is available, they work closely with the tyre manufacturers to get what their development team want. It's the vehicle manufacturers pulling the strings not the other way around, Pirelli, Mitchelin et al will produce whatever they need to compete for those volume sales.
It's academic anyway, that's a multi billion £ industry unlike the kit pen market who are never going to work together to standardize the components.
