So, why is that?
Well, at the most simple, communicative level, viewers often associate eyes with two circles on the face. That’s often a key part of drawing from a stylistic perspective—finding the balance that you like between communication and proportion. There is no right or wrong answer on that!
This idea of communicating in symbols is sometimes a barrier to overcome when trying to draw from sight (i.e. drawing what you “see” versus what you “know”). For example, look at the image below. We all “know” that eyes don’t actually look like this in our day-to-day lives. However, we recognize this as an eye. We know that this symbol represents an idea.