> I can not believe that aliens traveled several
> light years, arrived to Earth, and built massive
> but primitive stone monuments.
You are limiting yourself to only what we currently know about physically traveling from point A to point B. What we call "travel" could simply be going to the next "room" depending on the technology used. In fact, we we might stand still and the destination might come to us! Physical space is only one manifestation of the universe, just as we once thought that matter was completely particulate...until we discovered it's also energy. Let's not impose our own culture as a limit to all the still-unknown principles in the universe that our small brains composed of fat, water, and protein, are completely unable to fathom.
> I expect more
> developed alien technology, for instance, superior
> materials and machines. A skillful ancient
> engineer could easily move 100 tonnes stone block
> by ropes, bars, and muscle power.
The very fact that you "expect" something based on your preconceived notions is the reason you are unable to allow the possibility of any other intelligent being. Even Gene Roddenberry understand the principles of the non-invasive "prime directive" of not leaving any sign of your own technology behind when you leave the planet.
> Perhaps aliens
> have visited here, but I can not see any influence
> on technology. However, aliens seems to have had
> influence on art; there is old pictures of
> alien-like beings.
I respect your opinion, but that doesn't rule out such an alternative. For example, I can't see how ancient humans are obviously responsible for the bizarre rock formations in andesite that we see in the Peruvian Andes. It's one thing to realize they didn't have the tools to accomplish such work in igneous rock, it's another thing to realize the formations defy any logic according to what we currently understand about human thought patterns.
How can any of us ever know, when all we can do is think?