According to a dude named Neil the grass Tyson the reason that the moon perfectly covers the sun is because the sun is 400 times farther away, so it appears the same size as the noon.The diameter of the sun is about 400 times larger than the diameter of the moon, which means that the cross sectional area of the sun is about 160,000 times larger than the cross sectional area of the moon.
It seems like a pretty big coincidence, but Neil is an astrology expert, so maybe he knows.