But also to control their own ports and set their own tariffs so as not be exploited by Northern industrialists in the same fashion that British mercantilists once exploited the colonies. Also to enjoy benefits of their own wealth - at they time seceded the South was paying about 70% of Federal revenues, with less than half the representation. Sort of a "taxation without representation" situation. I can think of worse reasons to secede.
You must have missed the part where they used forced labor.