but not because they can't be. They aren't usually literate because few people spend the time to actually master make and explore how other projects have built up their make files to learn what makes an excellent makefile.

I would say that the terseness of the syntax and use of pipes makes it very literate once you know enough of the low level syntax of tools like sed, grep and awk to read them likes sentence fragments joined by pipes. It's parses very linearly as each line usually reads like a complete compound "do this, then this, then this" sentence describing a bunch of steps sequentially.