A lot of APIs don’t support the tooling you mentioned, have all sorts of rough edges (return improper http codes, or don't return them at all), or act...quirky at scale (based on my experience at an automation workflow org).
Of course, as APIs improve (OpenAPI, etc), the barrier to entry for all workflow tools declines (which is a good thing!). You're then just serializing and deserializing JSON (or a mutation thereof) between two or more steps. Just as Google makes their Terraform provider a first class citizen by autogenerating the Terraform provider based off of their APIs (Hashiconf 2018 SF talk), so could integration partners for workflow automators. Just takes time for the ecosystem to mature.