Because there's also the issue of the hard drive and if the rumors are correct, the new MPB will only have a solid state drive, which either means much smaller capacity or much higher cost. I recently replaced my 256GB MBP drive with a 750GB drive (which also has an embedded 6GB flash drive, which I assume is used for cache) and even with the drive shortage, I was able to do that for $200, including a case for the old drive, which I can now use as a portable external drive. In the current Apple MBP configurations, for the 15", if you replace the 750GB 5400RPM hard drive with a 256GB solid state drive, it costs an extra $500 for almost one-third the capacity!
Futhermore, to date, the MacBook Air specs have NOT met the MBP specs. That was the difference in the lines. I would not like to see Apple take backwards steps just to fulfill someone's idea of Zen and I don't want to be forced to do everything in the Cloud. There's nothing wrong with the Air, but the machines fulfill different purposes and I think Apple needs to maintain both lines.