No, you’re not missing anything - there’s not a way to automate that in Asana itself.
However, it’s definitely possible via my Flowsana integration, which has a “Copy subtasks from a template” rule action. You can see more about that here:
In fact, your specific use case - triggering subtasks to be added to a new task that’s created from a Form submission - is quite a popular usage of the “Copy subtasks” rule action.