While all/majority of AWS APIs requests are signed with HMAC, it is not as prevalent for other APIs in general.
Most likely, the reason is because of the gotchas involved when client has to make the signed API work as they may miss a few headers etc or new headers got added by proxies and so signatures don't match etc.
HMAC signed requests have clear benefits like there will be no secret is in transit and there has to be protection of the secret at rest on both Client and Server, though.
What could be a scenario where product teams should be enforced with signed API requests and not go with API key authentication , otherwise could be a high risk.