https://glitch.com/ can do static sites! It has a pretty good code editor with live editing capabilities! It has git import/export! Backends if you need em! You can remix any project to make your own copy and modify it!
(Mumbling about the problems below the break)
The reason why this is rare is because the costs don't work out. Glitch reached really far with the backend instances (which cost significantly more than static sites) and couldn't cover the costs of free backend instances with our premium product, although we also didn't really come up with a great shape of premium product to sell, either. Also we were constantly drowning under a flood of people spamming accounts to try and host free Discord bots and such, and getting that under control was non-trivial work.
But even if we had dropped backend support and gone static-only, the maintenance effort behind a good editor experience for a static site is way higher than you'd think. And that's only for an HTML+CSS+JS editor; WYSIWYG editing would take even more effort. Macromedia had 1400 employees in 2004 (most of which probably weren't working on Dreamweaver, but still a non-trivial amount). Paying even 10 people a living wage is a lot of money.
You could maybe get away by leveraging the uncompensated labor of open source and use in-browser VSCode, but the maintenance effort of keeping that working with your hosting setup is probably a few engineers-worth of effort. And that's still not considering the ongoing effort of keeping spam and harmful content off your platform.
Glitch now exists as a ride-along to Fastly after getting acquired, and being subsidized by a profit leader product is about the only way it can keep existing at its current level of quality IMO.
