The timing of std::cout output is strange

#include <iostream>
#include <thread>
#include <chrono>
using namespace std;

int main()
{
    std::cout << "1";
    std::this_thread::sleep_for( std::chrono::milliseconds(1000) );

    std::cout << "2";
    std::this_thread::sleep_for( std::chrono::milliseconds(1000) );

    std::cout << "3";
    std::this_thread::sleep_for( std::chrono::milliseconds(1000) );

    return 1;
}
  

I want std::cout to be printed every second.

But when I run this code, the result value 123 is printed at the same time.

Why?

And please tell me how to fix it.