The answer should be very easy to find but I'm having trouble. There are thousands of websites offering free websites, and many offering resell rights on it. However where does the original reseller get their website template from? A template takes hours of work from a professional so it's not cheap or easy to make in bulk.
People make them for fun, to start up a portfolio, or to promote their custom template making business. With many free templates, you'll see a line in the footer that reads "Web design by XYZ.com".
Normally template reseller get the full reseller right from PLR selling site. By PLR the reseller have right to resell or give away the template to mass public.
Many designers build a web template, they use the same template and swap out elements and change the CSS file to produce other templates.