I dunno, because it's vastly superior? Because some of us want the best? Because it costs money to refine an engine, as they have been doing for years and years now? Because time and resources spent building an engine could be time and resources spent making a better game? Because four years spent solely on making the game could result in a better product than splitting that time with engine development?
I dunno, I can't think of a good reason. But you're right, id be a poor businessman. I'd want to put out the best game possible using the best assets possible, and licensing the fox engine would (hypothetically) almost gaurantee that. But it likely wouldn't generate more sales, since we will all eat this stuff up no matter how crappy (currently) it looks.
I definitely shouldn't do business.
Edit: Was half joking about licensing the engine but.... you seemed to think there's no good reason.