This is just the era of AWS (and amazon APIs). A lot of them look like SOAPy xml rpc because thats how they started, based on internal service frameworks. Check out SQS or S3 for similar examples.

Around 2010-11 there was a shift towards RESTish structures and json for serialization. A lot of methods and payloads still feel SOAPy, but look like json. I think it was DynamoDB which had the most unfortunate API with a literal xml-json transform live in the API service.

And then somewhere around 2015 you started seeing more well defined RESTish APIs with better tooling and adoption for smithy (and some openapi, iirc).