There is a lot of evidence that humans built the pyramids, including the remains of the workers' houses, and Egyptian documents detailing some of the construction process. 
The slow accumulation of human knowledge needed to build the pyramids can be seen in early attempts to build smaller versions using different techniques for block-cutting and transportation.
Early Egyptian tombs were rectangular mastabas, these were improved upon by the Pharaoh Djoser (reigning from 2630 BC), who built early step pyramids. The Pharaoh Snefru (reigning from 2575 BC) attempted to create smooth-faced pyramids with varying success. 
The pyramids at Giza were a natural next step in the improvement of engineering techniques.
Experimental archaeology has proven that granite can be cut with copper tools in combination with either wet sand or quartz chips, things Egypt has in abundance. Reliefs in Egypt show the Egyptians using primitive drills. 
Following the principle of Occam's Razor, the simplest explanation is preferred when all other things are equal. The existence of aliens needs extensive proof before it can be given as a credible alternative to theories involving human tools.