thread stopped after detach

I use the codes below to test detach (in ubuntu18.5, g++7.5):

#include <thread>
#include <fstream>
#include <chrono>
#include <iostream>

using namespace std;


void long_process()
{
    ofstream ofs("test.log", ios::app);
    ofs<<"long process start" <<endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(3*1000)); //wait 3 seconds
    ofs<<"long process end" <<endl;

}

int main()
{
    ofstream ofs("test.log", ios::app);
    ofs << "main start" <<endl;
    std::thread th(long_process);
    th.detach();
    ofs << "main end" <<endl;                                                                                                                                                               }

But in the test.log, I only see 3 lines, it seems long_process stopped after detach.

main start
main end
long process start

I think the result of detach should be:

main start
main end
long process start
long process end

If I change detach to join, it works.

main start
long process start
long process end
main end

I don't know why.