The Pyramids are an engineering marvel which, to my understanding, until today cannot be duplicated who planned it? (I remember once hearing a super strange theory I was wondering if this "theory" is widely known.)
That we don't know exactly how they did it doesn't mean that we can't duplicate what they did. There are no exact descriptions of precisely how the great pyramids were built, and that means we will never know for sure how they were built. But it does not mean that they had some ancient and mysterious knowledge that has been lost, or that it was aliens, or anything else like that. All it means is that any theory we have is a guess.
It is important to remember that the Pyramids weren't just a sudden invention. Pyramids developed from other types of buildings over hundreds of years, and the building techniques with it. Many of the techniques developed would however have been forgotten when new technological developments made them obsolete. There is therefore nothing weird in that we don't know exactly how it was done.
Building a whole pyramid with the ancient technology would be so expensive to do today that it will never be done. But many experiments on a smaller scale have been done the last hundred years to test various theories. As such we now have a reasonable knowledge of how it was done, even though we will never know for sure.
The most important part to remember here that there is nothing that says that it wasn't doable with the building knowledge of that age. There is hence nothing mysterious about the building of the Pyramids.