I believe you have to be East of the Mississippi to be considered "Southern," and Texas is considered part of the Southwest. It's funny, because Midwest seems by some estimations to span the height of the country until it hits Texas, but maybe it depends on what part of Texas you are in. Whereas in the East, from top to bottom, it's Northeast, Mid-Atlantic, and Southern, no one calls Florida the Southeast, though technically it is.

Why is it not, for example "North Atlantic," Mid-Atlantic," and "South Atlantic"?

Or NorthEast, East and Southeast?

Or North, South and Mid?

Of course, some people still divide No'th from South by where the Mason-Dixon line is ...

Heck with it all - you want logic? Try some other science!