Because it was part of the art world of European culture to portray people like the dominant population of Europe at the time.
In contrast, though, the genealogy of Jesus makes clear that he was from the Middle East, with ancestors who were Mesopotamian (Abraham), Canaanite (Rahab), and Moabite (Ruth). It is possible that also that he has some "African blood," so to speak, in his heritage, since Solomon had many wives, and we don't know the heritage of all of them. There is also the possibility of other interracial marriages in the long line of his ancestry.
Essentially, Jesus was of mixed Near-Eastern heritage. There's every reason to believe he had normal skin tones for that part of the world (medium to dark brown), dark brown eyes, and dark brown hair. He had a beard and, being a lawful Jew, he most likely had payot (
https://en.wikipedia.org/wiki/Payot).
Since the average height of a man in those days was about 5' or a little taller, that was likely his height. Obviously we don't know anything about his weight.
There is nothing about his appearance that would have been considered unusual or striking (Isaiah 53.2). He looked much like anyone else in that era and culture.