There are many website formats, let alone special applications like landing pages, shops, blog articles, mobile formats. During the old (pre-mobile) Internet a site builder was expected to be a jack of all trades.
Bootstrap and Foundation can help both web design newbies experts get better and precise results in less time than with just using an HTML editor. Experts can focus on specialties knowing the the HTML standard staff (layout, grid, responsive elements) is handled. Beginners enjoy great results from the beginning and learn a lot on designing with less effort than by studying the HTML5/CSS3/JS science. For all users, communities are here to help, such as Boo
Wix is search much more than Squarespace
Website builders often indulge the user with great visual effects minutes after signing in. However, most realistic projects (and in all client projects of a web designer), sooner or later the designer would like to add custom HTML. At that point it turns out if a site builder fits to the author's needs. Website creation software normally offers an HTML editor. What differs is the extent to which HTML can be added, for instance, when it comes to external scripts or meta tags.
A clean code may not be necessary for a perfect display. However, sites with a messy, broken code (let alone semantic) mark-up cannot be evaluated as easily, and it's not only about Google, Bing, Yahoo, Yandex. Or, for instance, if meta tags are missing or poorly assigned (Open Graph, Twitter Cards), a page won't as shareable.
Omitted (negligible figures on this scale): drupal site builder, squarespace site builder, strikingly site builder
"It's marketing, stupid" (according to Bill Clinton)