Comment by easton

AWS has a video about it somewhere, but in general, it’s because S3 was designed in a world where not all browsers/clients had HTTPS and it was a reasonably expensive operation to do the encryption (like, IE6 world). SigV4 (and its predecessors) are cheap and easy once you understand the code.

https://youtube.com/watch?v=tPr1AgGkvc4, about 10 minutes in I think.