Drawing Outside the Lines

and on the Wall, and on the Floor...

My children are driving me crazy!
Why, oh why, do they feel a need to draw and color on things that are not used for art???!
The two of them have colored on the wall, on the floor, on bookshelves, on the toilet seat, on my mom's table, on the couch, on the curtains in the living room...

You name it, they've probably drawn on it.
And I've punished them, done time outs, made them clean it up, etc and they STILL do it!!
I don't understand why they KEEP on doing it, when my mom gets so mad, and I get upset, and they have to clean it up.

What is it that drives them to continue???

We've got princesses on the stair wall going down to the front door, pencil and pen scribbles on the table, brown lines around the bathroom, white and red lines on the bookshelves...