Is Greek mythology and ancient gods not just mythology?

One of these days I was thinking about Greek mythology and it is incredible how evil those gods were and how come ancient Greeks talked about them as they really existed. This is so strange that this fact in itself would have motivated a fierce research to try to explain it. But in fact, just the opposite occurred. People just assume it was all imagination and classify ancient civilizations as totally stupid, although ancient Greeks were far from being stupid. This is all very curious. And what about the crossing of "immortals" with mortals giving origin to semi-gods. It is strange that these later were not considered humans! Interesting. This looks so similar to DNA manipulation, to other legendary beings such as giants and other strange stories we hear about here and there that makes me raise my eyebrows...