Possibly because a border is part of a column so if you insert a new column adjacent to a border you have to remove the old one and redraw it which is pretty an

Possibly because a border is part of a column so if you insert a new column adjacent to a border you have to remove the old one and redraw it which is pretty annoying because its tucked behind a few clicks and menus. A line however floats relative to the columns so you don't have to remove/redraw it. At worst you might have to move it.