Plans are 10% of actually building something, at that level. Materials aren't legos, each product needs to be qualified and have its idiosyncrasies addressed. Even at the consumer level, when you change manufacturers you often have to retool your process because while the specs are the same, you're delivered something different.
Keeping technology at the same level requires ongoing engineering. When that engineering is removed, the technology decays. It very quickly becomes a matter of reinvention rather than rebuilding.
Something often forgotten when companies offshore production.
Musk actually said that technology does not move forward if there is nobody (or no company or no government) to push it forward.
The absence of a business case is not the same as the absence of technology.
Technology for supersonic aircraft has moved forward since the Concorde. Economically viable applications for that technology for commercial passenger transport have not, however, appeared. (Or, if they have, they haven't been recognized.)
Given their extreme noise, pollution, and ozone layer disruption it's certainly a good thing there aren't hundreds or thousands of Concorde-equivalents plying the skies today.
Technology is both economics and engineering. Just because something can be done, doesn't mean it should be done if no one is willing to cover the costs.
I don't think the Egyptians used magic, perhaps they knew a trick or two we don't know about, i don't think there's any writing left about the construction techniques.
The Egyptians' "trick" was massive amounts of human labor over a long timespan. Nothing you couldn't do a lot faster today if you were willing to spend the money (and manage to get the permits).
The construction of the pyramids is not impressive from a technological point of view, but perhaps from a logistical point of view.
It would cost something like $5 billion today and would take ~2,000 people ~5 years. Which is relatively speaking a lot less expensive. Put another way, Bill G could build ~17 of them in his lifetime and not be broke.
Awesome! What's at one time only the province of the greatest empire on the planet could become just one of many of a billionaire's projects. Let's scale this out from the present day with something of seemingly unattainable current value: How about The United States of America? Maybe several decades hence, a billionaire will have made himself the equivalent? Colonization of Mars, anyone?
Also, he's planning to spend your money on it, not his.
The point is that technology can be lost. Sure, we may have re-learned the technology in the intervening millennia, but we wouldn't have to re-learn it if it wasn't lost in the first place.