The world-view that "newer is better" is a modern one, dating back to the post-Medieval, Early Modern period. Before that, the major epistemological approaches (that is, approaches to what is "knowledge" and how it's acquired) were reversed: medieval learning was based on the system of Scholasticism, a system of learning where the basic, original texts (whether those of the ancient Greeks, the Bible or post-Biblical pre-Medieval scholars like Augustinus) were read and re-read, interpreted and re-interpreted and conflicts and contradictions resolved and re-resolved. In the scholastic world-view, older is better, because the truth is god-given and predetermined, and all we can do is find echoes of it and interpret them.
The switch, in the early modern period, to observation-based learning and the belief that new knowledge can be created using observation is a major paradigm shift in how people see the world, and is part of the whole transition that includes the scientific revolution, the "age of enlightenment" and other cultural and philosophical shifts of the time.
So, how does that relate to our fantasy worlds? The genre of fantasy is heavily inspired by European medieval romances, and it carries a lot of tropes that come with it - and one of those is the "older is better". It comes with the territory. In contrast, classic science fiction, which is heavily based on modern, scientific paradigms, usually espouses the "newer is better" trope.