[formerly_proven]

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_.

[degamad]

Bearer tokens have a defined scope, which could be used to limit functionality in a similar way to pre-signed requests.

However, the s3 pre-signed requests functionality was launched in 2011, but the Bearer token RFC 6750 wasn't standardised until 2012...