I'm pretty sure I set up most of what "Simple S3" using with Apache2 and WebDAV at least fifteen years ago.

Every month there's a post of "I just want a simple S3 server" and every single one of them has a different definition of "simple". The moment any project overlaps between the use cases of two "simple S3" projects, they're no longer "simple" enough.

That's probably why hosted S3-like services will exist even if writing "simple" S3 servers is so easy. Everyone has a different opinion of what basic S3 usage is like and only the large providers/startups with business licensing can afford to set up a system that supports all of them.