Why is this error shown when the parent exits before the child thread?

#include<thread>
#include<iostream>

using namespace std;


void hello()
{
        cout<<"Hello World\n";
}

int main()
{
        thread t(hello);
        t.join();
}

When I comment t.join() ,

I get an error

terminate called without an active exception Aborted (core dumped)

But why do main has to wait for the child to complete? Doesn't the hello thread have its' own independence?