In my opinion, the reason the Egyptians were so obsessed with permanence was because they worked and lived in this life for the afterlife. Everything they did was to prepare for death and the afterlife. They built temples, preserved their bodies, and wanted to make sure they came back for all their stuff. I think the obsession with keeping their stuff even after they died was due to the fact that they were one of the first societies to settle down. After thousands of years of wandering around and not having very much to claim as your own, it is natural to have an urge to collect and keep whatever you can get your hands on. For the Egyptians, they were proud of all the things they had managed to claim as their own and were very determined to keep them, even if they were not alive to protect them.
They lived their lives preparing for the afterlife. They built gigantic temples to house their bodies and belongings so they would have a palace when they came back. The afterlife was something to look forward to, as it is for most societies. If I were going to spend an eternity in the afterlife I would definitely want to have some of my stuff to pass the time.
I also think that they built these large temples as a way of not only living on in the afterlife but also living on in this life. By mummifying their bodies and having large tombs and pyramids, they would be forever here on earth as long as their monuments would stand the test of time. They would less likely to be forgotten if there were constant reminders of their presents.