So how/why did people think that way?
How? They copied aspects of Greek art and architecture in their own works, though their sculptors didn't vividly color their creations like the Greeks did so it looked rather lifeless by comparison. As I said before, they incorporated Aristotle's vision of the universe into their own along with other Greek notions of philosophy and science.
Why? Well, now you're getting into a psychological issue rather than one of history and linguistics. In short, people have a tendency to venerate old things that seems to be hardwired in there. The older, the better as far as that goes. As to why… I couldn't say.
However, again, the Greeks didn't just come up ideas, but created dialogues or logical arguments that explained said ideas. Since Plato recorded Aristotle's thoughts for posterity and was a pretty sharp guy himself, Medieval Scholars had difficulty picking out logical problems within the system, but just accepted the arguments as they were.
Technological advances eventually allowed people to make closer examinations of the universe and exposed the holes in his theories, but there really wasn't a way to do so before then.
So it was a sort of a combination of herd mentality, veneration of the aged, and a case of being starstruck.