I don't really get it why people find it so hard to believe that the Ancient Civilizations could have build these monolithic structures.

Technology does not mean just cars and cranes that we have today. Ancients would have their own ways to do it or should I say "their own technology".

And maybe, their tech was better than us. As we do find proof of their great creations and we also don't find our environment to be adversely affected, like we are affecting now.