Hi @Todd_McKernan and welcome to the forum,
You actually can have multiple levels of subtasks now. To add a subtask to a subtask, display the subtask’s detail pane and click on “Add subtask” near the bottom of the detail pane.
What you can’t do, though, is view more than one level of subtask in the list view; that might be what’s confusing you. You can vote for that capability here: Ability to work on all levels of subtask from List view
Without that visual capability, it does make creating more than one level of subtask less useful and sometimes confusing. (Some Asana users and organizations discourage the practice for that reason!)