The Egyptian historian, Diodorus Siculus, said that they Egyptians say that their houses are only their temporary lodgings and that their graves are their real houses. Tombs and objects within them are duplicates of the living world to be occupied by the dead or their life force. The afterlife was considered to be the permanent life and a continuance of the temporary life they experienced while they were alive. With this being the belief, art forms were created in a manner that expresses this. Egyptian art portrayed beauty, even in ordinary things. This suggests that the Egyptians were very happy with their lives and wanted that happiness to continue after they died. In many ways, the Egyptian lifestyle was perfect. Who wouldn't want that perfection to be known?
I also think that the Egyptians wanted to be easily recognized in the world. They were proud of their culture and heritage and wanted others in the world to know about their beautiful lifestyle. They lived in a very rich area and had easy access to the necessities of life which relieved them of many of the day to day stresses people experienced. Because their art was easily identified as Egyptian, others had no doubt as to its origin.
My thoughts are that the Egyptians experienced such a wonderful life that they could not fathom it ending even in death. They created permanent homes to ensure that the life forces of their people continued to have a place on earth after death. They believed the afterlife to be as spectacular as when the people were alive so they made sure the permanent homes were beautiful. I think that the continuance of the style was primarily for easy recognition by the Egyptian people as well as others. People find comfort among things they know. If their sculptures and banners were created in a familiar style, they would feel safer both in life and in death. Beside that, if its perfect, why change it?