That's because Rockwell was an illustrator who worked from photographs and did a lot of tracing.
He was doing Photoshop before there was Photoshop.
Rockwell was a real artist who was equally at home drawing and painting from life as he was using photographs. Good illustrators use photographs as just another tool of the trade. The key to Rockwell's greatness (and to any artist) was his ability to design the painting. Arranging values, shapes and colors within the space to produce a great painting is only achieved from talent and years of experience.
Using photos and tracing will never turn a bad artist into a good artist. The same principles hold true for good photographers vs bad photographers.