Some painters are happy leaving it at that. Creating the illusion of three-dimensional space on a two-dimensional surface is, indeed, an accomplishment. It can be a very satisfying one, especially if the painting looks nice on the wall and can bring the moment alive to the viewer.
Of course, most of us want to advance beyond that. We really do want to be artists, and not little factories turning out home decorating accents. (Not to belittle the craftsmen; I, too, enjoy looking at a nice picture.) We want to dig a little deeper, discover something unique about ourselves and express it in our own way. We want to prove our existence - prove that we're alive, and also prove worthy of being alive. We want to prove that it's not about the money.
Here's a knife painting I did not too long ago. I was playing - playing with color, playing with form - and just exploring a bit.
Munds Mountain Snow, 12x9, oil