Because the alternative is forcible oppression and denial of the right of self-determination.
Read more about the right of self-determination here: http://www.unpo.org/article/4957
Read more about the right of self-determination here: http://www.unpo.org/article/4957
Perhaps, if that is what the people in the South (and not just those in positions of relative legal privilege) wanted.
But I doubt that, for example, the 55% of Mississippi's population that was in slavery was particularly energized about secession to preserve the right of their masters to keep them in slavery, though.
Edit: they've neve been independent, that's more of the bs that we're hearing these days.
I think you may be confusing the pattern of colonization in British North America with colonization more generally; the pattern elsewhere was more often of subordination to an imported elite rather than displacement.