Can't one get their prenatal vitamins from fruits, dark green leafy vegetables too? Fish etc

Yes. You certainly can. The only reason why such supplement is needed is due to the fact that many people do not consume a healthy enough diet and thus the supplement acts as an insurance policy. Regardless of what diet you adopt, be sure that it contains enough folic acid, calcium, and vitamin d3.
Prenatal. In addition to eat well balanced diet, i will suggest you still to take prenatal multivitamin to ensure your folate, (folic acid) iron and other nutritional component are adequate for a healthy baby and healthy mom. Although you get majority vitamin and mineral from your food, however during pregnancy, there is a significant increase of demand of those nutritions- related to the fetal growth and development.