This makes me think that some of this practice might have been motivated by professional pride on the part of the mapmakers. Such as, "oh, the only reason I didn't go farther was because of the ravenous beasts, and my rival would never be able to push the boundaries farther either so you might as well buy/trust in my mapmaking"
You may be right, but I'm also inclined to include that it's fun to draw monsters.
Another month, another rationality quotes thread. The rules are: