I've hear this criticism leveled at Picasso, but Picasso *could* draw; he'd have been a great realist if he wanted to be. As you begin to study the things that set a good painting apart from a mediocre one, you start to see that not all of them have to do with accurate draftsmanship or the subjects chosen. You begin to pay more attention to the interplay composition, balance, geometry, and shading. These are things that exist *apart* from the things represented; they can exist *without* anything represented at all.
Once you've reached the level of appreciation that understands what sets apart a great painting, it's natural to then begin to focus on those things. That leads to stripping away things which matter not at all (subject matter) or somewhat less (fine detail, accurate perspective), and gradually you find yourself on the road to abstract art. It is this process of thinking about art that I suspect confers cognitive benefits on art students.
Picasso's landscapes are a good place to start to see this process at work. My favorites are the Factory at Horta de Ebro and the Reservoir at Horta, which you can see are still quite representational without being realistic.
Now what the *market* will pay for a piece of art is totally irrelevant to what art is. But a single white line *can* be art. The problem is that you've come in at the end of a long conversation about "what is art?" You've missed everything but the exclamation point at the end.
Think of it this way. I can buy a quality blue crew neck tee shirt for about eight bucks; something you could wear with jeans and look OK. But decorate that tee so it looks like an old London police box, and suddenly the market value goes to $20. Why? We all know what the tardis looks like on the outside. Because it makes you part of a cultural conversation.
The objection to a white stripe being art seems to be, "I could do that, where's my 44 mil?" Without defending the prices paid at auction for any particular piece of art, some of which clearly is driven by greed and irrationality, what collectors are paying for is the exclamation point at the end of the conversation. Sure you could paint a white stripe, but you weren't part of the conversation.
Suppose our Tardis tee shirt is one of a kind, hand-printed and signed by Tom Baker. It goes at auction for $100. Does that seem irrationally high to you, given that you can buy an equally good tee shirt for $8? If I put *my* signature on a Tardis tee shirt, that would actually *lower* its market value, but the signature of one of the actors who played The Doctor *raises* the market value. Does that seem irrational? If you'd never heard of the doctor, science fiction, fandom or TV for that matter, it *would* seem crazy to pay $100 for a tee shirt with a sharpie stain on it.