The perspective in this isn't all that bad... my credit to him goes out for both shear size of it (40"x20" the size of an actual painting), hours spent, and attention to detail. Digital painting is a true pain in the arse. Overall he has done a really good job folks.
The reason? Why even question that... photoshop is just another medium, like paint, pencil, plaster, photography, and so on.... asking why is like saying why paint at all when there are cameras (which actually has been in debate off and on in the art world since cameras were introduced, more or less its impact but still). Just my 2 cents...