Dynamic languages such as JS allow you to override almost anything. It’s essentially impossible to sandbox “part” of JS code, you must either sandbox the entire runtime (e.g. in browsers) or allow everything (e.g. in node.js).

Individual packages want to run arbitrary JS code to do “safe” things during installation. But it’s very hard to allow them to do anything meaningful like file access, even if it’s “safe”, without potentially allowing exploits.