It’s easy to
romanticize medieval artists, but it’s important to remember that these are differences in style and technique, and not necessarily because the medieval mind was more "childlike" or open to a certain viewpoint or less developed.
Linear perspective was not discovered until the early
15th century, by an
architect named
Filippo Brunelleschi. It is linear perspective that creates the illusion of
three dimensionality in a flat surface like a
canvas. You can see this technique being developed over a number of years in visual art, from the stumbling application in early
Renaissance art to its perfection in later Renaissance
painting.