I just see anyway of making a one size fits all driveshaft, there must be over two dozen different implements that can be put on a tractor. One option is all battery power implements, like a batter powered hay bailer to start.....
And then you have the optional 3PH QA 'systems' available (JD iMatch, Pat's, etc...) that add more length to the shaft.
And then each tractor itself is different... as an example a "Cat 1" tractor can be multiple HP's = size and height, what tyres are mounted, length of 3ph arms?
There are just too many variables and configurations to have a 'standard' length(s).