First, as >>2474
said prepping isn't necessarily bad, in fact I also agree that it's beneficial. But that's besides the point.
AFAIK, whoever the current generation of humans are have thought, more or less, that they were approaching the end times. I don't really know why. If I had to guess, it's probably just a really dang useful survival mechanism. I mean, it encourages forethought and preparing for unpredictable calamity.
Also, we now live in a world that changes far more rapidly than any other generation has experienced. Previously, such as the 1200's or something, one could expect 100 years in the future to be mostly the same as the current time, with only a few drastic changes to lifestyle. Now even so much as 20 years can have wild changes to how we live our lives. We're also not super great at accurately
predicting the future. So for many, a far easier to understand and simpler prediction of how events will unfold is societal collapse followed by a rebuilding period, rather than the strange twists and turns history will actually
take. Even actual collapses of nations have never been as cut and dry as some doomers would make you believe.
Another major contributing factor is various forms of media. Legacy media (news) makes its money by getting people angry and afraid. Plenty movies, shows, games, etc. have there setting in an apocalypse or about the inevitable decline of our world. And soy-boy social media provides an excellent echo chamber for making things seem worse than they are. Basically some assholes make their living by doom-preaching.
I think we're also more aware and/or willing now to admit the ways in which industrial society sucks, which probably contributes something to the feeling of impending doom.