Because none of them
know your brand.
Squarespace and Wix have great editors. They have terrible blank pages. They ask you to already know your brand before they can help. If you didn't already know your brand, you wouldn't be there.
GPF's site builder reads your foundation and starts the page filled in. Hero, story, services, hours, sign-up, footer — all in your voice, your palette, your type. Then you edit, like you would anywhere else.
Update your foundation later? The site updates too. Change your archetype? The site re-skins. Add a new menu item? The block adapts. The brand and the site are the same object.
Publish to a custom domain. Export as a WordPress theme (yes — a real, plug-and-play WordPress theme). Or hand the code to a developer.