To put the age-old UCS argument aside, considering the unnecessary height added to the actual model by its inclusion (resulting in aesthetically unpleasing deviations from the dimensions of the source material), couldn't it be argued, in a way, that the 10221 Super Star Destroyer's playset interior is, in fact, visible from the outside? With the current trend of ever-increasing accuracy in UCS and exclusive sets 'larger than minifigure scale', one can't help but expect that the set designers would have placed aesthetics and a reasonably strong resemblance to the source material above play features in the making of such a set (especially with something as large and unwieldy as 10221 – more likely intended as a display model than a toy in the fashion of 10144 or 10195).
In response to your preference for a UCS Falcon with an interior, while such a feature would have been wholly possible, I can also see why The LEGO Group may have decided to pass it over (aside from the obvious reason of price, that is). For one, there's the fact that the Millennium Falcon does not actually have a true, working interior – the layouts depicted in the Incredible Cross-Sections, the Technical Journals, the RPG, and many others all disagree with each other significantly, in addition to shrinking down or enlarging various components in a way that may come across to certain fans as inconsistent with the nature of the craft. This means that regardless of which layout they went with, the set designers would ultimately have failed in creating a representation that appealed to the Star Wars-centric AFOL collectors at which it was obviously aimed.
And while I would not have had any issue with displaying a 'cutaway' Falcon, one could also consider the possibility that The LEGO Group thought it less than worthwhile to put effort into an interior that would have remained hidden for most of the set's time on display (an argument that could also be made in defense of 10174 and in further opposition to the playset interior in 10221). A similar case can be made for 10221 – as well as the fact that no interior for the Lambda-class T-4a was actually shown in the films (meaning that the inclusion of any interior would prove confusing to those who had not expossed to Expanded Universe sources).
On the other hand, to use my demographic argument again, the case could be made that Star Wars-centric AFOLs with enough interest to spend $200+ would in fact have enough knowledge of Expanded Universe sources to recognize an interior for the Lambda if it was included – but the interior is ever so plain...