Same reason the Moon is made of green cheese: it is not. In most cases the default format is some kind of localized string. Sometimes ISO format is used but usually with dashes for better readability. YYYYMMDD(or %Y%m%d in strftime parlance) is seldom the default. To be fair I am sure I have seen it but I cannot think of an example right now.
Unix date (GNU core utilities)
date
Output:
Wed Sep 26 22:20:57 CEST 2018
Python
import time
print(time.ctime())
output:
Wed Sep 26 22:27:20 2018
C
#include <stdio.h>
#include <time.h>
int main () {
time_t curtime;
time(&curtime);
printf(ctime(&curtime));
return(0);
}
Output:
Wed Sep 26 22:40:01 2018
C++
#include <ctime>
#include <iostream>
int main()
{
std::time_t result = std::time(nullptr);
std::cout << std::ctime(&result);
}
Output:
Wed Sep 26 22:51:22 2018
Javascript
current_date = new Date ( );
current_date;
Output:
Wed Sep 26 2018 23:15:22 GMT+0200 (CEST)
SQLite
SELECT date('now');
Output:
2018-09-26
LibreOffice Calc

Gnumeric

OnlyOffice

Python + numpy
import numpy as np
pd.datetime64('now')
Output:
numpy.datetime64('2018-09-26T21:31:55')
Python + pandas
import pandas as pd
pd.Timestamp('now', unit='s')
Output:
Timestamp('2018-09-26 21:47:01.277114153')