os.system calls run like serial execution multi-threads

My co-worker asked me why his code cannot run in concurrency in multi-threads. I discover the os.system function is acting weird unlike the other functions in multi-threads. I code a small demo to reproduce the problem. Appreciating somebody who can answer my question.

Run like serial execution

import os
import time

from concurrent.futures import ThreadPoolExecutor, as_completed

executor = ThreadPoolExecutor()
begin = time.time()

for _ in as_completed([
        executor.submit(os.system, cmd)
        for cmd in ['sleep 10', 'sleep 3', 'sleep 2']
    ]):
    print("completed after", time.time() - begin, "seconds")

output

completed after 10.005892276763916 seconds
completed after 13.017478942871094 seconds
completed after 15.034425258636475 seconds

Run normally

Replace os.system with subprocess.call.

import subprocess
import time

from concurrent.futures import ThreadPoolExecutor, as_completed

executor = ThreadPoolExecutor()
begin = time.time()

for _ in as_completed([
        executor.submit(subprocess.call, cmd, shell=True)
        for cmd in ['sleep 10', 'sleep 3', 'sleep 2']
    ]):
    print("completed after", time.time() - begin, "seconds")

output

completed after 2.019604206085205 seconds
completed after 3.017033338546753 seconds
completed after 10.009897232055664 seconds