With well known shows like Grand Designs often showing people trying to build their own home, without a project manager, you can see why the temptation to manage their own home’s construction would persist among many would-be home-builders, but there are a number of things which are just out of reach for laymen; somebody with no experience in the construction industry, and an inability to work on the project full time.
Construction project managers have a number of advantages over a laymen, beyond simply understanding how to build things. They also have an understanding of the construction industry, which means that, for them, hiring/finding high quality contractors, getting building materials, and more, is all a lot easier. Lastly, even if you don’t need somebody responsible present throughout the project, having somebody who can dedicate their attention to an issue at the drop of a hat is something which is not possible for your average person getting their home built.