Because shell scripts are more complex to write. I, myself, having used grunt and then switched to gulp, ended up favoring shell scripts / npm scripts, only to

Because shell scripts are more complex to write. I, myself, having used grunt and then switched to gulp, ended up favoring shell scripts / npm scripts, only to realize gets much more complex for larger projects. I think a build system abstraction is legit, and Fly is pushing the status quo to an even simpler, thinner, yet flexible alternative.