I can live preview my website from my local server / computer / laptop, while writing content from basically anything. Even a cheap and underpowered writing deck with 400 MHz CPU. The options are limitless.
You're absolutely underestimating the complexity of proper live preview of changes. This is essentially "hot reload" mode, but on the public internet, because it has to run on a public domain. Getting that right is a challenge, and if you don't know why, you haven't attempted to solve it yet.
Why would I want my WIP site to be on the public internet? This has been built into Jekyll for years. Probably other SSGs too but I don't know/use them.
Because you're thinking in the context of a solo developer working on their site locally. You don't need a CMS. People that want to collaboratively work on a website, some of which may lack technical skills, need a way of previewing their edits that doesn't involve running shell commands.
* the ability to schedule posts
* a ton of plugins
* a lot of people who know how to use it
* a reasonable WYSIWYG interface
As far as I know, most SSGs fall down on one or more of those dimensions.