This is done only by those who wish this to be so.
"let's not sell our ancestors short.... why must we so arrogantly stick to the idea that our present civilization is the pinnacle of human development?"
With the increasing skills and understanding of archaeologists, geologists, etc since the 19th century, this view is much less prevalent, isn't it? New evidence and dating methods make assessments of how long humans have been in different areas of the world much more secure; blood types and DNA improve our understanding of links between peoples, so that there is no need of ascribing skills and achievements to aliens of any sort.
"certainly there is much evidence in mythology and on the ground that a civilization more advanced than ours existed.... lots of myths about the Gods descending and performing wonderous acts.... but, again.... where is the proof that the Gods were alien? we have no solid proof that Aliens exist even today...."
This is conjecture, being replaced by better evidence and fact Mythology is evidence only of the infinite ingenuity and capacity of human minds to create stories. I do not know of any that cannot be put down to this, do you?.
"personally, at this point I tend to lean towards the belief that so called Aliens are creatures from another dimension.... ..."
Could this be because you prefer a good story to the lack of evidence? The knowledge that it is an idea, a story is, in my opinion, a much more exciting concept.
"...and as for the giants.... we DO have actual fossil evidence that these beings existed.... but like the Neanderthals and other humanoid species, the giants are now extinct.... "
"in my opinion, not only is the Discovery Channel pushing such ideas as AA for entertainment and profit but also have a more sinister ulterior motive.... to instill the concept that Aliens created us and should be worshipped as God..."
Well, I am pleased to say that the idea that they might, just possibly, succeed in this apparent aim is almost non-existent, but if that is their aim, it should not be ignored, but counteracted with facts.