Thats true, but wouldn't it be still required if you're having a internal S3 service which is used by internal services and does not have HTTPS (as it is not exposed to the public)? I get that the best practice would be to also use HTTPS there but I'd guess thats not the norm?
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.
Because a bearer token is a bearer token to do any request, while a pre-signed request allows you to hand out the capability to perform _only that specific request_.
Theoretically TCP packets have checksums, however it's fairly weak. So for HTTP, additional checksums make sense. Although I'm not sure, if there are any internal AWS S3 deployments working over HTTP and why would they complicate their protocol for everyone to help such a niche use case.
I'm sure that they have reasons for this whole request signature scheme over traditional "Authorization: Bearer $token" header, but I never understood it.