Companies that provide programmable systems should be the first to provide SDK's, even for technologies not yet widely adopted. That goes for any company that provides something programmable.
Amazon appears to have no trouble at all justifying the massive development effort required for every new back end web service they build, but front end programming API access just isn't valued as much.
Honestly, is that really a surprise at all?
REST web services work well. Most languages have frameworks with excellent support for them.
Historically, the community has provided wrappers for popular APIs pretty quickly.
In the specific case of Go, it's still in early adopter territory. That means they are unlikely to have a problem using the raw APIs.
Their back end services make them money. Creating a Go API does not.
Makes it hard to justify investing development resources.
I did this in rough a week and a half of full-time work.