Well, my grandparents built with absolutely nothing in their pockets and did almost everything themselves. Back then, building materials weren't that expensive, and of course, they went without a lot for a long time.
You can still do that today; but many don't want to anymore or don't dare to. Or they have different priorities in life.
And as for the cost of building materials: wages were correspondingly lower as well.
Also, people today often don't just want a house; they want a certain standard.
There is also a trend toward more square meters per person. In other words, smaller, cramped old buildings are demolished and replaced by more spacious living units. In the end, there aren't really more living units available.
Furthermore, there is still a certain urge to move from many areas in East Germany and elsewhere to the places-to-be.
Secondly, you would then have masses of vacancies somewhere. At some point, the government will intervene...
Last year, Postbank presented a map/statistics showing where they see potential for positive value development until 2030; but also where things are clearly going downhill.
The question for the original poster is whether you live in an area marked more in green or rather in red.