They had 100s of years of practice though.
Have you ever been to Egypt? You can walk around certain tomb complexes and see all the beta versions. Took them a long time to get it right.
You start with these long flat tombs called mastabas, then they started stacking them on top of each other, so you end up with a step pyramid.
From there you can see how they'd want to build a proper pyramid, but it took them a while to get right, so you have some unfinished versions out there and things like the bent pyramid. Eventually they worked it out and you get Giza.
You can go out there and see for yourself. It's easy to get a driver and see all of these transitional phases in a day, and when I went there were some really well preserved mastabas you could walk around. Completely destroys all the theories when you can see all the transitions for yourself.
Have a look at a better explanation here:
https://gilgamesh42.wordpress.com/2012/09/15/ancient-aliens-the-pyramids/
I think I this is the best explanation so far. Not sure about taking 20 years (think it's longer)
How many people in here have been to Egypt?
Divers even uncovered a papyrus in the Red Sea.
https://www.haaretz.com/jewish/archaeology/1.754616?v=D3FB3361B9261484C5EFAD205C75C50ABut the picture makes sense. I agree there must be also unfinished versions like beta pyramids too. Hence why all the Pyramids in Giza aren't the same size. I think it's plausible for the smaller pyramids to take 20 year's, but not sure about the bigger ones.
Some believe if you enter the pyramids you are cursed.
Maybe not a 1000 years, as I admit now that is exaggerated, but I do think the bigger pyramids alone took more than 20 years. The one that was built by Pharoah Khufu stands today at 455 feet/ 138 metres tall.
That Pyramid is the one known as 'The Great Pyramid'.
