We know the physical limitations of the Ancient Egyptians. They worked with copper tools, had no pulleys or other simple machines which made the medieval cathedrals possible, yet their civilization left monuments over forty stories high.
A lack of information on how they accomplished this feat has led some to conclude aliens were responsible for these marvels of antiquity. It's understandable, in a way. The civilization studied doesn't seem to have the capabilities to create this artifact, therefore look for another civilization responsible for the item's creation. This path is useful if you want to explain how a necklace made from material only found in South America was uncovered at Cahokia Mounds (it was transported over trade routes) or how steam-engine technology developed in England suddenly appeared in the conquered territories of its empire. The conclusion that aliens built the pyramids is the interweaving of a pattern observed in the interactions of human civilizations with concepts from the space age. It makes an exciting story, but it lacks material support. It is myth, not theory.
Suggesting aliens built the pyramids requires an alien race to have interstellar travel...yet even with this advanced technology, still be dependent on human assistance...to gain human cooperation or compliance, they must expend effort at communication and physical control. After accomplishing all this (for reasons unknown) these aliens would leave without further trace or impact. Such a theory requires an inordinate level of faith and dismisses evidence in support of competitive theories. It is more implausible, demanding greater evidence than the initial theory that the Egyptians themselves built these structures.
But in emphasizing the terrestrial origins of the pyramids, we must note they are not simply the result of human ingenuity, though they attest to the power of ideas to unify people in the pursuit of a resource-hungry goal. They stand as a testament to states and religions, the technologies of complex ideas. However they are also products of the elegant and simple evolutionary algorithm.
The Egyptian climate acted as a selective pressure on beliefs about an afterlife and resulting funerary practices. Pit graves in the desert evolved into mastabas, perhaps influenced by grave robbing and/or drifting sands which would have obscured smaller burial sites. (This would be the economic/social climate exerting as much selective pressure as the geographic climate.)
Imhotep innovated the first known step pyramid by stacking consecutively smaller mastabas on top of each other. Egyptian creation myths may have laid the foundation of his idea. Alternatively, the optical illusion of the sun rising or setting in the desert appearing to form a step pyramid may have also inspired him. Though we memorialize Imhotep and the Egyptians deified him, had he not existed it is possible another would have innovated a similar step pyramid. It is the social, economic, intellectual and material environments which play the greatest role in innovation, not the brilliance of the individual.
More remarkable still is the intense growth and innovation of pyramid construction in the following 100 years. It is comparable to the innovation of high-rise buildings and skyscrapers in the modern age. Snefru commanded the construction of the Pyramid of Meidum. It was a step pyramid with a limestone casing forming the structure of a true pyramid. This design was quickly eliminated by the selective pressure of gravity.
Next the Egyptians tried a pyramid with an initial design angle of 55 degrees. When gravity once again threatened with structural failure, they adjusted the angle to 43 degrees, which left the legacy of the bent pyramid. However, Snefru was not deterred. Perhaps he was spurred on by the allure of immortality or religious devotion. Perhaps he had political ideas of consolidating and centralizing power. It is this adaptation, this response to natural limitations and motivations which drives life and technology forward. Under his reign the Egyptians attempted a third design, successfully completing a true pyramid.
His son, Kufu built the great pyramid at Giza, yet the full articulation of Ancient Egyptian theology and the resulting innovations in burial customs and tomb decoration had yet to evolve. This evolutionary history was obscured by desert sands and time. Persistence rewarded investigators with the answers to the origins of the pyramids. And yet these answers provoke more questions and a drive towards even more investigation.
Do not simply dismiss the idea of alien pyramid builders. It is an interesting study of the process of human inquiry complicated by the limitations of human perceptions and assumptions. It is a complex story of the interbreeding of ideas. For more in-depth discussion of the evolution of the pyramids, see Pete Vanderzwet's article The Evolution of the Egyptian Pyramid.