What happened was that the fiat currency got disconnected from gold. We got off the gold standard, and that was the beginning of the downfall. Greed, quantative easing, loans upon loans upon loans, bankrupt countries, war expansion to keep the dollar standard running while gobbling up resources and economies, the works.
Read this article WTF happened in 1971 (and why the fuck it matters so much right now) https://cointelegraph.com/magazine/2020/09/24/wtf-happened-in-1971
Most of the currencies are today worth 2-3% of what their worth was back in 1971 when they got disconnected from gold. Paraphrasing @saifedean. Think about that for a second and please consider getting at least a bit of Bitcoin.