Why dont early paintings tend to look very realistic

Because during that time people, artist, were mostly monks or people especially trained with early learning of art. the art of anatomy and realism just became a thing. they also only had so much resources to make the paintings.

because they didn't have much refrence as to what realistic was.

pitch- the definition of pitch is as follows:

"the quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone."