The Image is Set in Stone
The idea of Jesus as a bearded Caucasian was universally accepted by the middle ages. The depiction of him as the good shepherd fell y the wayside and the more recently accepted robed and long-haired, and bearded version has remained as the most popular version since then.
Poetic License
The historical painters could not find descriptive passages in the bible to guide them when they painted Jesus so as artists do, they used their imaginations coupled with their own ideal of their Messiah and related that to previous artworks already on a show to the populace to generate similar works.