That's because southerners are liars and idiots. They started the war, so it is a blatant lie to say "war of northern aggression." Since they argue that Lincoln tricked them into attacking, they should call it the "war of southern stupidity."
Well, it's not that simple.
Their view is that they 'just wanted to leave the US', making their land not part of the US.
So, they said, no, US, you can't keep having forts in the south, and said that the people in the forts had to return to the north and could not be resupplied.
Lincoln defied them to resupply the fort, and they attacked the fort, which was now in 'their' land. Their intent was not to attack the northern states - so they view the war as caused by the north. And it is different than if they'd attacked northern states.