The Egyptian alien engineer thing is mostly a meme, but it stems from the fact that Western Egyptologists can't get their heads out of their asses long enough to agree on how it was done. Plus the Egyptian government won't allow any kind of physical or non-physical investigations because it would conflict with Zahi Hawass's personal pet theories.

No technology was really lost, we just can't agree on how they stacked the stones into pyramids. The stonecutting technology they used is very well understood and we even have real actual tools used by the actual ancient Egyptian stonecutters and some of the tools used to manipulate and place stones.

There's also several incredibly solid theories on pyramid construction, but to verify them would require investigating the actual pyramids, which Hawass will not allow.

We could “easily” (in technological terms) build the pyramids today, but nobody wants to spend a $100B on a 150m-tall pile of solid, imported rock with 3 small rooms in it.