The easiest reason for me to explain is that no two people's feet are really the same size or shape, and that to do so, means that you will be making a shoe that is made for someone else, even if their foot length happened to be the same as yours. Not even in the Middle Ages were two matching shoes necessarily absolutely identical, since they were made for different feet, for different people. The same "style" of shoe made by two different Cordwainers may look completely different.
More than that, though, is that any leather garment, worn for any length of time, changes shape, so that the "pattern" soon ceases to bear anything but the most cursory resemblance to the original, which means that, among other things, areas that in the leather original had been stretched thin and flexible will not be thin and flexible in the new item (thus suggesting that your slavish reproduction is anything but).
Finally, of course, is the fact that these are archaeological remains, and may have stretched, shrunk, twisted and deformed in ways that are subtle to the eye, but when replicated into footwear may cripple you. The designs in this document should be suited to present the general designs, just as items such as The Tailor's Book present general designs that show you how a certain piece was supposed to look.
The link above has some valuable info and suggestions for anyone interested in making period footwear.