The problem with this build, or any other build I come across is that it assumes we already own a windows computer, which is just not that common of a thing these days.
I don't even know anyone with a windows desktop computer in their home at this point, some have a windows laptop, and most are fine with just their phones and tablets. So no we don't already own a copy of windows, or a mouse, or a keyboard.
The price listed is essentally the cost of an upgrade, rather than a new computer.