Because it has the resources to alter the footprint of its products with such frequency that third-party creators can’t possibly keep up.
Because consumers will stop buying from said creators due to frustration in having to ditch a case when they pick up the latest iThing.
The Smart Cover appears as if it will have a long shelf life, because it doesn’t enclose the iPad. Apple can simply watch sales of the ubiquitous cover continue to grow as it delivers newer iPads that still make the accessory useable. I’m betting it is working on a similar “case” for other iThings.
Apple doesn’t want its users tainting the “look” of its products with a case that hides the “look” inside. So it is developing its own solution, and essentially doing what its users often revere it for: innovating something that already exists by re-imagining it.
It’s a new take1 on the old saying that if you cant beat ’em join ’em. Apple’s version reads like this:
“If you can’t beat ’em, make ’em obsolete.”
1 Or re-imagining.