Just because we don't know doesn't mean there was anything extraordinary happening.
The roman colosseum built in Rome is considered to be on par with the pyramids in Egypt and they were built many decades after wards but it is an awesome feat of engineering. Running water up 6 stories for the lower caste to drink from and a ceiling that could close over to keep out the hot sun and rain. It was built I think over 8 yrs and it is believed to be able to house around 50,000 people. That's bigger than a lot of stadiums today.
You give any empire a large group of people and 20 yrs to build something by hand and it will be marvelous. All I am saying is Egyptians built them and we aren't sure exactly how? All this means is either we are either shortsighted or don't give them enough credit. It is a very simple structure that is just enormous. I would like to think if any "ET's" were involved it would be something that is anything but a simple structure on a large scale.
Aliens could have built it but I would rather think that humans by themselves are intelligent enough to build these mountains of stone. There are many ancient structures on earth that we aren't sure how or why they were built but because we don't know exactly how doesn't mean we had help from a more intelligent life form. All this means is that the knowledge is lost to us through the passage of time or we are too shortsighted to see it and would rather give something simple a lot more credit than it deserves. By simple I mean why and how it was made.
The Mayans created there calender by keeping track of the stars through observation and they(planets/stars) are probably some of the most constant entities in the universe unless something catastrophic happens like a supernova or a black hole disrupts their travels. I don't by any means think this is easy but a whole group of people studied the stars and they tracked their movements. If one single man did this than it would be something on the verge or miraculous but a lot of people did this over a long period of time.
They did have early ideas of steam power at the great library in Alexandria back before 0AD. There are shipwrecks in North and South America that suggest there were Romans or Carthaginian here thousands of yrs before Columbus or even the Vikings. There is as much to suggest the world was much different in ancient times than we believe. They say for every animal killed in the Colosseum at least 10 were killed getting them there. The great Aurochs of Europe and Africa were made extinct by the games. Gengis Khan wiped out whole civilizations because they betrayed him. He destroyed almost everything that was a record of them.
The Human race is a powerful powerful force and it was no less so 4000 yrs ago. To say that humans couldn't have built these great structures because we don't understand doesn't mean "ET" helped it means we usually find it easier to take something amazing and insult ourselves saying we couldn't have done it.
Alexander the Great took over his known world and it was a massive 5.2 million square km. and he only lived to be 33. He didn't rule until he was 20 yrs old and his empire still affects western civilization today. That is amazing and no aliens came down to help.