The simple fact is that pen kit manufacturers are no more likely to standardise on a few standard sizes of tube than car makers are to simplify the wheel sizes of cars so there are only three or four sizes of tyres needed.
Even if they standardised tube sizes then the varying external sizes of components would mean that various sized bushes would still be needed.
This type of restriction would prevent manufacturers from producing the wide variety of kit designs we currently enjoy.
Perhaps all nuts and bolts could be made the same size so Scotsmen only ever need to buy one spanner. 
