I've seen other, photo quality pics done using Photoshop/Illustrator... and its not about creating a photorealistic scene from scratch because someone hired you to do it, but because you want to challange yourself.
It is an Art, because one has a passion for it, and they strive to better themselves artisticly (even if to some, it may seem a waste of time).
By the sound of it, the image was composed piece by piece, then put all together in one file, so I imagine, it was not as computer intensive as one imagines (though, you probably still need a hell of a rig.... and a damm good backup.. can you imagine working on some fine details for 5 hour's, then bam... your computer crashes... yikes...)
Very impressive though...
Shadehunter