Or bodies are in a constant state of getting older and undergoing collapse. I think that believing in the good old days is a reaction to getting old. I think that believing in some golden past is it reaction to our own bodily degeneration. Fear of our mortality is a powerful force, and I think that a large amount of people externalize/project that fear onto their perception of society.
The end of the world hasn't happened for everyone yet, but the world does end for some individuals every day.
Reminds me of the poem the florist has in Grim Fandango that goes something like: it may be years, it may be hours, but sooner or later everyone pushes up flowers.
I think it's more that you get taught the "right" way of doing, speaking, etc. and people are geared to dislike challenges to that idea until they learn to accept change. Another example would be people who've learnt how to do a particular task at work being shown a better way of doing it but having a niggling sense that the way they'd learnt first is ipso facto better.