@MissDaphne - I thought that, too. I was really resistant to giving my little ones vitamin D drops because I thought my milk should have enough.
But my pediatrician explained that babies aren't designed to get vitamin D from breast milk - they're supposed to get it from the sun. But now we know that too much sun is bad for your skin! So babies who do not get much sun, especially those born in the winter, may need to take vitamin D supplements. Vitamin D is not expressed much in breast milk.
Since the Industrial Revolution, most of us have needed vitamin D supplements since we're not out there working in the fields any more! Vitamin D started being added to the milk when people suddenly staying inside a lot and rickets became a huge problem because of the vitamin D shortage that causes.