The advantage is that you can focus on what you want. Sometimes you just can't get a photo of what you want from the angle you want it. Or you can't get the right things in the frame. Illustrations offer more options. The Millbrook Press specifically targets the educational market with their books.
Scientists are people who notice details.
They measure, test predictions and keep on trying. Photos show children doing the things scientists do. It's difficult to present scientific discussion for such a young age group. Strange Nests by Ann Stephens is an example of Millbrook's supplementary curriculum material. The author is a birder.