> Aren't paintings and screenplays typically sold to someone else for a fixed price after they're created, assuming someone buys them?
Yes, the difference here is that if you write a novel, create a painting, make a sculpture etc, and your target client doesn't buy, you can shop around for another buyer.
I'm not really sure there's a market out there for "book cover for a Tim Ferriss book" other than Tim Ferriss himself. This is the problem with spec work of this variety - you have one winner, and a bajillion losers who will now see absolutely nothing for all the work done.
> how should they get started?
At a shop, working under someone more reputed than they are, until they have a large enough portfolio of their own to show potential clients. That's the ideal scenario anyways...
You can apply the same thing to programmers, though. Say you have an aspiring hacker with no references or previous code to show you - how should they get started?