If that's all you want to do (concat, minify, convert es6/sass), then by all means, I do agree make is the simpler albeit slightly less maintainable and non-cross-platform solution. I use make for my quick static sites too.

But when you need more features like bundle splitting (app code and libraries live in separate files, for caching), cache-busting, lazy-loading, separate configs for production and development, etc. make doesn't quite cut it IMO.