I'm no expert, but as far as I can tell the precision of the Great Pyramid's construction is the most impressive ancient engineering feat/inexplicable mystery that could have only have been achieved with extraterrestrial interference here. Aren't the lengths of the sides consistent to one another with an error no greater than an inch? I think the base is supposed to be level to a similar degree of accuracy. Whichever way you cut it, that's damn impressive and I'm not sure their surveying and setting out techniques are fully understood, but that doesn't in any way necessitate alien involvement, it just means we still have to work it out.
There are several prehistoric constructions or beliefs that we are yet to properly interpret, and maybe never will; Stonehenge for example, a far less impressive (though still incredible in its own right) monument is also poorly understood - we have a vague understanding of its phasing over several centuries culminating in the monument as it is today, but we still don't know exactly how the Welsh bluestones were transported there, or even what the bloody thing is actually for, and probably never will.
I've seen a few estimates that seemed fairly kosher that suggested that it could easily have been built in a couple of decades or less, especially if they had a workforce (slaves or skilled craftsmen) that numbered in the tens of thousands, potentially. Personally I find it fairly easy to believe that a pharaoh would be able to mobilise such a sizeable workforce. He was, after all, a living god, and it seems like he had a massive fuck off ego too.
The more prosaic explanations do not in any way detract from the sheer incredible magnificence of it all, and other buildings and cities from ancient Egypt and other cultures around the world. Anyone here been to Angkor Wat and the surrounding temples? Fucking incredible place. Humans can do some truly mindblowing stuff, when we put our minds to it.