Once you start caring about cross-platform, incremental, distributed builds with a bunch of people committing to a monorepo those advanced features start becoming attractive. Shell scripts handle the 'run this build/test task' but not 'pull these artifacts from another set of tasks, run this task to produce a bunch more artifacts, cache them for later.'

In these scenarios the lines between build system, package manager, and task runner all become pretty blurred. Haven't found an all in one solution that I would call "portable" yet.