Because business is all about revenue, sales and profit margin. If they lose market share in the small tablet size to all the new displays (Nexus 7 seems most popular now, but Nook HD is a contender with its awesome display and Google Play now enabled), they'll have to do something to keep up spec-wise and use the Apple eco-system to differentiate. Falling behind in hardware purposely to protect another product line or segment of the same is poor business.
Companies want profit and sales wherever they can get then. To skip out on the form factor sales potential by going with retina and instead just staying the same is like giving up. Sure, sales are high now, but in your line of thinking why ever add more CPU power or memory/storage? That would also compete in the iPad4 range. If they don't keep up with current technology and surpass where they can, they lose.
There's a reason we don't all drive cars from 1974 and the reason is not because a company was afraid their innovation would harm sales of their other vehicles. Companies adapt to markets and demands. If one model or size sells far less than another, that's reason to reevaluate if you should even be selling it.
