Why was the Egyptian art in tombs 2d and not 3d or frontal portraits?

Answer 1

Ancient Egyptian depictions of people were strictly governed by tradition and religion, as well as purely practical considerations.

The rule was that each individual part of the human figure must be shown from its most recognisable viewpoint, even if this meant showing the eye (for example) as if seen from the front, but the rest of the face is seen in profile (from the side). Look closely at ancient Egyptian paintings and you can see this is true. An eye seen from the side would have been more difficult to understand.

The same applies to shoulders - they make most sense when seen from the front, but the rest of the torso and legs are always shown from the side. This does not mean that ancient Egyptians walked around in a peculiar pose - they walked exactly like everyone else.

Frontal portraits are rare in Egyptian art, but they do exist. They are usually reserved for musicians, acrobats and other entertainers, who were not thought to have any status - so it didn't matter if they were shown differently.

Painting "three dimensionally" on a two-dimensional surface is all about shadow and highlights, light and shade, tonal modelling and understanding perspective. None of these things were known to Egyptian artists, who continued the same flat style of painting for thousands of years and never felt any need to change.