I used to work at a bike shop and thought it would be easy to build a bike from the ground up. Easy. Get a frame, fork set, and some other parts. Easy, right? I had no idea the amount of small parts one needs, including all the other minor stuff you have to buy like brake lines, headset, etc. Instead of a few week process, it became a few month process because I had to keep buying all these things I had forgotten about and didn't know I needed.
Now scale that up a house. I can't imagine the amount of things one could easily forget about on a project that scale and not realize until it was too late.
And yes, my grandparents in Europe also build their house like you say. Not being involved in the process (nor other family that I know of), I’m afraid that such knowledge isn’t being passed down.
But I agree: It’s easy to greatly underestimate the amount of work that goes into a house. Contractors will have entire crews, all of the equipment they need, and most importantly they have years of experience to know how to do something quickly and in a way that will pass inspection.
A DIYer must first learn each step before doing it, which can take as much or more time than the task itself in many cases.
I'm not sure what you have in mind here, but my brother did an "owner build" in the true sense of the word and sunk every nail into every piece of wood, every pipe, every wire, every window, floor, staircase, insulation, lighting, built all the cabinets, everything. That was 8 years of his life. Two of those years he literally quit his job and worked on it full time.
Caution, it is easy to seriously underestimate how much work it is to build a house. It could stretch on for years. Not to mention the pain in the ass that is bad weather, the logistics of sourcing and moving all the materials, plans, permits, etc.