Actually it wasn't too long ago, in the early-2010's, that Microsoft was promoting SharePoint for internet sites; I think at one point some Europoean car manufacturer (BMW? Ferrari?) had their global marketing site on SharePoint. Of course that didn't last long, as Microsoft licensed it at a crazy price ($40k per site or something like that).
I worked on a couple of public facing SharePoint 2010 sites for large, well known companies before while it was in RC and immediately after - MS had a big marketing push to get people to build more than Intranet portals on it at the time. It seems like that died off entirely once Office 365 came around, and it was never a good idea in the first place, but it was definitely a thing.
The Navy still runs more than a few web servers using Sharepoint, albeit behind dedicated network firewall appliances.
The Secretary of the Navy's page (at https://www.secnav.navy.mil/Pages/default.aspx) for instance, is a Sharepoint site. I used to maintain a Navy website hosted under there, and had a bunch of Hugo-specific scripts to convert a Hugo static site into something I could upload to the Sharepoint and have it mostly still work (which involved things like rewriting links and renaming files to end in .aspx).
Well, they would have had to purchase one Client Access License per potential device or user that would access the website. Since there's about 5-6 billion people with internet access, and a CAL is about $50 a pop, that would be roughly two hundred and fifty billion dollars to fully and correctly license a public server.
And it probably needed a very hefty bunch of servers, even after caching, if you needed just a little bit of dynamic content or interaction with the site.
