Given the historical context (S3 launched 12 years ago, 5 months before EC2 launched with the us-east-1 region), it's reasonable that S3 buckets were global because regions didn't really exist yet as a concept.

If you look at the docs now[1], new buckets are regionalized and the region is in the URL for non-us-east-1 regions.

[1] https://docs.aws.amazon.com/general/latest/gr/rande.html#s3_...