There is no real answer yet, so in the meantime aliens is a viable theory.
Since there's no evidence, the following are also all 'viable theories':
-Time travellers from our far future travelled back and oversaw the construction of the pyramids
-The Pharaohs really were gods.
-The pyramids built themselves out of nanotechnology gifted to them from a parallel universe, when they finished assembling themselves, the nanotech went dormant and the silicate turned to sand.
-The pyramids were built to hhouse a race of subterranian dragons who kept the people of Egypt in terror.
-The Egyptians themselves were more advanced than we are now, however as their society fell into decline they hid all traces of their technology in order to prevent it falling into primitive cultures.
-The pyramids have always existed, they are the survival capsules launched by a race before our universe even began, when the Earth had cooled sufficiently, they left their capsules, breaking the protective force field and seeding the Earth with pre-big bang life.
-The pyramids were built under advisement from the greatest minds of the ancient world, from Greece and China, providing the mathematical skills for them to build these structures to a greater accuracy than we would expect.
This is why we wait for evidence instead of accepting avery half-baked idea which comes along and granting it the status of 'theory'