People don't feel rich. In many families, both partners have a job. Median incomes didn't increase much in the last 10-15 years. With an after-tax monthly income of 5,160€ as a family (or 3,440€ as a single), you belong to the top 10% [1] (translates to roughly 100,000€ / 70,000€ pre-tax anually). The median monthly net income is 1,869€ (roughly around 35,000€ pre-tax anually).
At the same time, people are highly suspicious of the stock market. People have less time for their kids, which spend more time in the educational system. So people wonder if the educational system is really that great for their kids.
I think this is interesting. It's like a vicious cycle: According to [2], 23% of Germans own stocks (although I remember that this number used to be around 15% a few years ago). So, people read books about how everything is going to crash down soon, because they're suspicious of the stock market (and to be able to say "told you so" afterwards). At the same time, these books reinforce the idea that owning stocks is risky.
Most people would rather buy a home, but houses got very expensive in cities in the last 10 years. In major cities, a house for a family of 4 would cost around 800,000€ and more, which is quite expensive for the low salaries of 90% of the people.
Then, major newspapers now have articles about the climate crisis every day. There's the uncertainty of the Brexit. There is uncertainty about immigrants, but also uncertainty and fear of the rise of right-wing parties in Europe (and the AfD in Germany).
So, we have books about how "the system" is shit, how "the crash is coming", how "Germany gets dumber all the time", how "something needs to change". Add some fears about the before-mentioned topics and there's your new book every few months, same content, different cover.
[1]: https://www.zeit.de/wirtschaft/2019-08/institut-der-deutsche...
[2]: https://www.zeit.de/wirtschaft/2019-08/geldanlage-sparen-ver...