Question

why is it difficult to create accurate maps of Earth?

Answer 1

Maps depend on points of reference. Even trigonometry depends on having the right measurements, whether it's in angles or in distances.

So, until the world agreed on where longitude started, there were many different systems, each accurate as far as the users were concerned. The only problem was to have an accurate measure of time, and that was not possible until the 18th century.

Anyway, most maps before that time were extremely local, and they were reasonably accurate as far as they went.