S3 was one of the first services Amazon Web Services provided, and they wanted a global namespace for simplicity. The first, and then only, S3 region was "US Standard" which is what's now us-east-1. They've since split it out into each of their regions, but kept the global namespace for backward compatibility.
Amazon keeps a lot of things for backward compatibility. S3 happened to be an early wild success, so they had a lot of users using it. Changing it to require specific regions would take a lot of work. Instead they're opting to follow a strangler
pattern, where certain features are only available when you use region-specific URLs. However, they're keeping the global namespace for... backwards compatibility.
"With great responsibility, comes great paperwork."
- From the SOC2 Testament, Book of Traceability.