I think I found the answer from the comment of the source code here:

// Because forking processes from Java is expensive, we prefer to launch a single Python daemon, // pyspark/daemon.py (by default) and tell it to fork new workers for our tasks. This daemon // currently only works on UNIX-based systems now because it uses signals for child management, // so we can also fall back to launching workers, pyspark/worker.py (by default) directly.