The evidence is that no one has a tablet with size similar to 7.9 inches with resolution approaching 2048x1536 without weighting a tons.
And costing a fortune.
Why not build the proposed direction for the device to start rather than producing, then obsoleting a display after only a few months.
Because (a) it's not just a few months, (b) the cost of stalling on the mini any longer exceeds the expense of retooling for a new model (which they'll have to do anyway within a year), and (c) the
current mini's display is already more expensive than the competition and they need time and volume to bring down the costs on the touchscreen components.
For timing, let's say they are ready to scale in August 2013. A hypothetical iPad mini with the display couldn't ship until around November, and that's being pretty aggressive. For comparison, the displays in the current mini have been in volume production since at least early September, when the first parts leaks started to happen, and that was with an established process.
Further, with the Kindle and Nexus sales numbers, Apple just couldn't afford to wait any longer without risking a serious uphill sales battle. Contrary to what is often said, Apple has never shown a desire for 100% market share in anything (in fact if that were their goal, they'd make some very different product decisions), but they also won't bother on a market they can't be a significant presence in.
Cost-wise, it's just not there yet, and the GF2 touch panels, with their high cost and yield issues, along with the need to go full-blow GOA due to the slim frame, is just asking too much. $80 is quite a bit more than competitors are spending even on higher-density displays, and so they need time to bring the touchscreen part down $5-10, time to wait for panel prices to fall so that they could get the retina for ~$30 more, time to get the A6 hardware down quite a bit, and probably time for some battery engineers to do some magic.
Just look at Google if you're doubtful. They launched an aggressive Nexus 10 with a retina-class display. Wouldn't they have done the same for a 7" tablet if
anyone could have supplied it, especially since they're not locked into a double-resolution model and could have gone with 1680x1050 or 1920x1200? It's not ready yet at any price, let alone one that works for the 7" market, even with an at-cost sales model.