Because Larry,
it takes time to do all the details, and time costs money. Most draw a model boat, with enough to get the outlines right, and then assume the modeller will have photo and other evidence to fill in the gaps.
Draughtsmen, like James Pottinger, draw the original boat, and record as much detail as they can as a truer record of the original boat. Usually, but not always they are also paid to produce them, where as the model draughties usually work for themselves or other model business', hence the limiting budget.
kiwi