I say: f*ck the mods.
I'm trying to stay below Strelecz35's number of warnings. I seem to collect them like Zelensky collects presidents as friends.
Poland has similar demographic problems now as most European countries
That's the bottom line. It really is not very exceptional, just a country with a typical post-industrial fertility rate (France is probably the only big exception, but it's also playing catch-up).
There were a few waves of demographic change in the last two-three hundred years. In pre-industrial agricultural societies it made economic sense to have as many children as possible. However, mortality rates were also extremely high, and as a result population growth was stagnant or modest. Then, with the first improvements in public sanitation and healthcare that arrived around the same time as the Industrial Revolution, life expectancy exploded and child mortality declined, but birth rates remained the same as before. The result was a huge explosion in population. Life had changed, but people kept breeding like it had not. This is what is happening in Africa currently.
Several factors worked to correct this situation. The mechanization of agriculture meant less hands were needed for work. Public education regarding contraception was also important. Urbanization, however, was probably the primary driver. Each successive generation that was born and lived in a city would have lower and lower fertility rates. In cities, kids went from being a productive asset, to a net drain on capital. Because they needed to go to kindergarten, school, uni - meant that their labor was no longer available, and instead you had to invest in them over decades. This made urban families change their calculus and lower the number of children to 1-2, since a larger family meant poorer outcomes in both education and career for the children.
The present day problem is the astronomic cost of raising a kid, that makes people not want to have kids at all. I think this explains, to a certain extent, the huge fall in fertility after 1991 in most Eastern Bloc countries. The state used to offer those services for free, but now they became a serious financial burden on the parent.