Partly because painting was all there was - no photography, TV, radio or the rest of the list. So art and music were the entertainment of the day. The printing press was nearly 1500 and it took a while for books to get around after that. Some art was educational, too. So, many paintings were "pictures". Not serious, not portraits, just pictures. So girls were a "a girl", dogs were "a dog" and much of everything was just fantastical - made up in the imagination.