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)