Imagine we wanted to make an exact copy of the pyramids in Egypt, the great pyramid specifically.
Never mind building them without machines. Do you think mankind, not just the current residents of Egypt, but do you think in this world there exists the technical know how and political will to even be able to do it?
If you think the only legacy of the roughly 3,000 years of Egyptian civilization is the pyramids, then you're sorely mistaken. Even if it was just the pyramids, let me tell you my friend, were man to disappear from the earth tomorrow, the pyramids would be proof of intelligent life on this planet for another 50,000 years.
A single reason? Why would we spend billions to do any great thing when it's all dust and ashes in the long run.
What else could be undoubtedly accounted as ancient Egyptians legacy? Name something we widely use today. It should be easy to do, if I mistaken.
We spend billions to build megalitic projects even right now. Electric grid is providing electricity, railways connect infrastructure, HPPs for producing energy, giant factroies to make goods and so on. A lot of reasons to build megalitic projects for billions.
But what's the use of building a pyramid?
What a logical fallacy. Just because the pyramids are the most famous of the legacies of the ancient Egyptians, it doesn't mean that it's their only legacy. It's like saying Napoleon was the only famous person from France because he is the most famous.
What's the use of Mt. Rushmore? The Colossus of Rhodes? The Acropolis?
May be a fallacy, but you still didn't name anything significant we use now that could be named a legacy of ancient Egypt civilisation.
No use at all. But civilisations who build them leave not only that artifacts, but also many other useful knowledge and legacy we use everyday.
It seems you've answered your own question with your second response.