Except for the Amarna Period, Ancient Egyptian art maintained the same style for thousands of years (…and that's a long time, considering that art styles later on—like in the 19th and 20th centuries—would change every few decades). Why did the style not change over time?
Archaeologists discovered, painted on a wall on an Ancient Egyptian temple site, red lines that formed a grid. Researchers took that grid and applied it to other Ancient Egyptian murals, finding that the proportions matched each figure identically. The Egyptian artists had been using a grid system to dictate their design of the human body. Like the law-driven society itself, the art was forged through order and consistency. Other grids were found on even more Ancient Egyptian finds, showing that all the drawings had been carefully made with exact measurements and specifications. The grid system was an essential part of Egyptian culture that lasted throughout all three major kingdoms in the civilization's history. It's really quite extraordinary: if you look at images from the Old Kingdom, they will look exactly like the art from the New Kingdom, some three thousand years later—no change. Compared to other civilizations, Ancient Egypt was one of the most static empires in history, and the grid system is one of the evidences for it. Talk about sticking to tradition! (Because artists in the future will come to mock traditional styles and will invent new ones). The Egyptians kept the same style for thousands of years. I marvel at how that could be so, but it's not that easy to look up more about. Sources are few. A good one that I recommend, for more info on the Ancient Egyptian grid system, is the PBS documentary How Art Made the World.