Non-experts build their own houses even in developed countries. For example there is a subcommunity of folks helping each other to build DIY houses out of straw bale https://www.homebuilding.co.uk/ideas/67k-straw-bale-self-bui...

Self-sufficiency and other "simple" things are possible in developed countries, they are just intrinsically hard and this is probably a good limiting factor because basically lots of things (eg foraging) don't generally scale if a lot of people want to do them.