Hi I'm an engineer on Actions, and a few months ago I probably would have said the same thing, but I've come around to it (and then some).

We spent a ton of time working through the syntax changes in HCL, YAML, and other languages. What we found was that while HCL and other languages are amazing for expressing pure configuration, software development lifecycle and continuous integration workflows are a blend of configuration and pseudo-scripting, and we were only able to come up with a clean syntax for this in YAML.

For example, although HCL supports heredocs, we felt that they suffered from readability problems versus multi-line strings in YAML when you're just trying to read through a workflow file and see what it does.

There are many, many other reasons we felt that YAML was actually the right choice, and I'm optimistic others will come around to it in the same way I have.