When they make XML do these things (or the way Github Actions does it with YAML), they're essentially creating a representation of the AST that the compiler would make internally from a mini language. So there's a few possibilities:
They don't know how compilers work and reach for a tool they do know
They know, but figure the problem at hand doesn't need the complexity of a mini language and start the project the quick and dirty way, and it gets out of hand as they add features
They may or may not know, but they do get caught up in the hype of some other tool (likely what happened with XSLT)