Which Egyptian empire do you refer to? Humans have been living in Egypt for thousands of years.

Follow up question. Was Egypt ever technically an empire? I've heard it referred to as such quite often.

I don't know too much about Egypt, but it seems like it never got very large, such as the Macedon/Greek or Roman empires.

The first part of your question is a very difficult question to answer, as there were anywhere from 3 to dozens of Egyptian empires depending on if you count new dynasties and foreign rulers as Egyptian or not.

The second half is actually a bit easier to answer. First, I should note that for most common people in the countryside (which I guess isn't the best term as for all of its history to the present day 99% of the Egyptian population has lived within a few miles of the Nile) while things like language and clothing changed very, very slowly, until recently when the Nile was dammed and the population urbanized, day to day life probably didn't change too much. The poor didn't have elaborate tombs and carefully mummified remains, and Coptic, the largest language in Egypt after Arabic, is actually a direct continuation of the Ancient Egyptian language just with some Greek influence.

As for why the "big" traditions like tombs and the old gods aren't practiced anymore, the simple answer would be monotheism and time. Even after the Greek and Roman conquests, many people continued to follow the older traditions for many reasons, including the eternal practice of "because it looked cool". In fact, even by the New Kingdom many tombs were being built purposefully to imitate tombs from older periods in history, so people have been nerding out about Ancient Egypt since slightly-less Ancient Egypt! Back on topic, though, by the time Egypt was Christianized many of the older traditions had been lost simply due to time, and the adoption of monotheism accelerated this, but the older tombs and temples were mostly just left alone and its probable that like in Europe the common people continued to worship older gods for centuries after the elites converted. The Islamic conquests brought with them an even stricter monotheism that led to many temples being taken apart for building materials. Eventually the old traditions fade from history, though its hard to say exactly when.

Still, its worth noting that while they may not be worshiped anymore, many Egyptian gods had followers for over 3,000 years. That is an absurd amount of time, longer than any living religion today, even Hinduism if you want to go all the way back to the earliest rocks with etchings on them from 4000BC, and like Hinduism the actual religion changed drastically over time.

Follow up question. What Egyptian empire was last to use the hieroglyphs and what happened to them?

Actually Egyptians still celebrate holidays from ancient times. Sham El-Nessim is an ancient Egyptian tradition celebrating the springtime. However it's celebrated similar to how many nonreligious Americans celebrate Christmas; only the secular aspects are practiced. This is obviously because Egypt of today only has two dominant religions, Christianity and Islam, both of which prohibit polytheism, or belief in other gods.

Follow up question. To what extend does the current population of Egypt descend from Ancient Egyptians? My understanding is that they Arabs, i.e. descendants of invaders from the Arabian peninsula.