Because things have been on a constant downward spiral for multiple decades, if not centuries, interspersed with the occasional improvement. Technologically we are improving, while simultaneously declining socially. The symptoms of that are post-modernism and its consequences, which only accelerate the decline.
This social decline is why media is mostly getting shittier, because in order to make meaningful & engaging art that will stand the test of time, it requires someone to understand core ideas/universal truths/concepts about the human condition. And those are slowly being eroded to a point where people (overall) have no true concept of things such as respect, honor, community, friendship, love, progress, etc. but only a perversion of such.
The people, at any point in this process, will perceive the past relative to them as better, because in some ways it objectively was. The same way we look back on those that came before us, so will the next generation look back on us.
TL;DR: Things haven't been good for a while, but are definitely better today than they will be tomorrow. Nostalgia is a symptom of a declining society. And yes, we live in it.