Because they just want the latest and greatest, never mind that you won't be able to tell the difference on a 27" display compared to the current model. I won't pay extra money for it.
This.putting Retina on an iMac 2011 27'' is just cool than a real need
retina display on small monitors I dont understand. If I could get a 32" iMac with retina display (or larger), then that´s one step further to my dream machine. It would be a huge display right in front of my eyes, and even if I would only use a small part of the screen for a program such photoshop it would still be sharp and big enough - that´s one step closer to a minority report computer, were you have a large display in front of your eyes and you can just shuffle things around as you please and have multiple stuff running at once. ....that would a perfect iMac