This is a large part of the reason. Money that gets earned gets spent pretty quickly on one thing or another, and it would be next to impossible for a family to

This is a large part of the reason. Money that gets earned gets spent pretty quickly on one thing or another, and it would be next to impossible for a family to save up enough to buy all the materials to build a house in one go. Another reason is that they don't have easy access to credit markets, so they can't just get a mortgage and move into a home and start paying it off. One more part of the reason is that they usually build the homes themselves, and its rewarding to see the house go up brick by brick - visual progress is being made. I'm not an expert, but these are my informed observations and learnings.