You'd have to build an access road to get over there, which would have to cut right by the Grand Floridian and be 2-3 lanes in each direction. Then you'd need a multistory garage that didn't obstruct any sight lines around the Magic Kingdom, with extensive on/off ramps and an area for tollbooths. The Universal parking garage & access roads cover over 2 million sq ft and are, what, 50-60 feet tall?
To cover 2m sq ft, the parking garage would have to extend as far north as Big Thunder Mountain on the plot you're talking about, and guests parking there would be close to a mile from the entrance (can't easily do trams in parking garages, you'd need a huge system of... moving walkways? people movers? a monorail?!).
The siting of MK makes a walking-distance parking lot near-impossible due to its frontage on the Lake and the siting of the Contemporary (which covers almost 2m sq feet and would probably be a better location for a garage). It just doesn't work unless they allowed a back entrance to the park, which would probably kill the souls of Disney fans who want that iconic Main St - Castle view.