Why are functions run sequentially and not in parallel?

Question

Python

My challenge is to run two functions at the same time. I used threading library. After launch, the main window opens, but another function (get_mail_func()) is only run when the main window is closed, at the very end. What is the problem?
Here is a code snippet:

if __name__ == '__main__':
    
    get_mail_class_start = get_mail_class(imaplib.IMAP4_SSL('imap.gmail.com') )
    root = tk.Tk()
    db = DB()
    app = Main(root, '**********')
    root['bg'] = '#2A3132'






    def all_tkin():
        app.pack()
        root.title('Main')
        root.geometry('1000x700')
        root.resizable(False,False)
        root.mainloop()
    
    
    init_app_thread = threading.Thread(target = all_tkin(), args = ())
    init_mail_thread = threading.Thread(target = get_mail_class_start.get_mail_func() , args = ()) 
    init_app_thread.start()
    init_mail_thread.start()

Thanks in advance!