Because JavaScript never had threads so I/O in JavaScript has always been non-blocking and the whole ecosystem surrounding it has grown up under that assumption

Because JavaScript never had threads so I/O in JavaScript has always been non-blocking and the whole ecosystem surrounding it has grown up under that assumption. JavaScript doesn't need a GIL because it doesn't really have threads. WebWorkers are more akin to multiprocessing than threads in Python. Objects cannot be shared directly across WebWorkers so transferring data comes with the expense of serializing/deserializing at the boundary.