It seems more of a cultural issue that -- I'm pretty sure -- predates Microsoft's acquisition of GitHub. I assume crappy proprietary yaml can be blamed on use of Ruby. And there seems to be an odd and pervasive "80% is good enough" feel to pretty much everything in GitHub, which is definitely cultural, and I'm pretty sure, also predates Microsoft's acquisition.

GHA is based on Azure Actions. This is evident in how bad its security stance is, since Azure Actions was designed to be used in a more closed/controlled environment.