Maybe the real question here is why we don't use standard parsers everywhere all the time? Well, for some data formats we usually do - take JSON for example. (You can do a lot with jq.)

I think the problem is that for most languages, the AST is very complex, and writing a transformation pass isn't something you do as a one-off hack.

Lisp famously keeps things simple. I think we could do better than that on syntax, but designing a language with an AST designed for hacking still seems like a good idea.

But there are other ways a language can be easier to hack on. The gofmt tool can do a search and replace on Go expressions and also write the output in Go's standard format. This builds on the language's strengths: (1) having a standard format (2) mostly unambiguous references to imported symbols, so you don't need type-checking to disambiguate. This makes it much easier to find all references to a function.