When the main thread returns (i.e., you return from the main function), it terminates the entire process. This includes all other threads. The same thing happens when you call exit. You can avoid this by calling pthread_exit.
The purpose of pthread_detach is to make it so you don’t need to join with other threads in order to release their resources. Detaching a thread does not make it exist past process termination, it will still be destroyed along with all the other threads.