Are you talking about stones like this?
The Romans were able to move stones that big with ropes, stones, and wheels. Why do you think people could not have moved these things? The entire architecture around Baalbeck, looks like ancient Greek architecture.
1. The fact that we don't know how something was done does not mean that it was done by aliens.
2. Any civilization that can navigate the positively enormous distances necessary to arrive at our planet wouldn't need to navigate via landmarks once the arrived. However, this does not mean that aliens didn't build the pyramids only that I disagree with the argument.
3. Much of the mythology of the Old Testament came from the Babylonians, Summerians, etc. The fact that these stories have been passed down through the ages neither affirms nor disproves their veracity.
I work with a woman who came here from India, and she says the people of India more or less take the fact that we have been visited by aliens for granted. I believe there must be intelligent life somewhere else in the universe, but I'm skeptical that we will ever come in contact with those beings. Most people focus on the incredible distance in space that would be involved in making contact. Also, species evolve and become extinct in a relatively short period of time, and this proccess is repeating itself throughout the very long history of the universe. So there is also an enormous distance in time that makes contact unlikely.
I'm not sure interaction with more advanced civilizations would be good for mankind. Everything we thought we knew would become obsolete or irrelevent. Man could no longer be governed by the old rules, the old ways. It would be chaos, and we'd be reduced to being pets of the alien civilization. And that assumes that they are benevolent. We could wind up being exterminated like the Native Americans.