Still in 2018 threads in Python do not make CPU bound tasks faster

sample.py

import time
from concurrent.futures import ThreadPoolExecutor
import random

start = 0

def print_time():
    global start
    end = time.time()
    print(end - start)
    
def fib(n):
    if 2 > n:
        return n
    return fib(n-2) + fib(n-1)

def run():
    items = list(range(1, 25))
    
    with ThreadPoolExecutor(max_workers=len(items)) as p:
        
        def threaded():
            global start
            start = time.time()
            print('thread', list(p.map(fib, items)))
            print_time()

        def single_thread():
            global start
            start = time.time()
            print('single', list(map(fib, items)))
            print_time()

        functs = threaded, single_thread
        
        for i in range(50):
            for f in random.sample(functs, len(functs)):
                f()

run()

"""
Controlling for the order of the functions and the process being warmed 
up the same code runs in about the same time regardless if it is threaded or not.

Last ~7 iterations example output
single [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.0468602180480957
thread [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.04687857627868652
thread [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.04687833786010742
single [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.046877384185791016
thread [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.046872854232788086
single [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.06251001358032227
thread [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.04687786102294922
single [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.06250452995300293
single [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.04687786102294922
thread [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.0624997615814209
thread [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.04687786102294922
single [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765, 10946, 17711, 28657, 46368]
0.06250953674316406
"""