Because I don't see any value from using it. It's not easier to read or write, it doesn't force you to keep it sane and simple like a bash/cmd script generally would, and it's just another dependency I don't really need. If I take a minimalistic approach to build systems, I prefer to go all the way.

Also, although this is merely a personal quirk that shouldn't persuade anyone else, I've seen enough horrific, unreadable make files to instinctively dislike them by now.