Possibly, the reason for lack of "empty spaces" is the 4e assumption of Dungeon Tiles or other types of gridded map rooms. While this can lead to slightly artificial encounters, it does mean that spaces are either "encounter" spaces, which are relatively small and expressly designed to host encounters, or "narrative" spaces, which do not need to be mapped, and can be described and designed by the DM.
For the most part, WOTC has done a good job with the quality of maps for their published adventures (I'm pretty unhappy with Trollhaunt). These maps can be scanned in, blown up to proper 1" scale, and tossed on the table, or recreated with Dungeon Tiles. However, throwing large caverns, halls, valleys, and other massive spaces into those maps would not allow them to be easily represented by Dungeon Tiles, or by color photocopies.
Of course, if such spaces were scenes for narrative, or even for skill encounters, you have no need for actual maps of them. Sure enough, such spaces are absent. I can see why the large "empty spaces" aren't included in their maps, but I still wonder why the writers don't spend more time on helping you with the description and narration of such areas. There's a bit of in in Thunderspire, where they tell you a bit about other areas, but you are left to flesh it out yourself. I think that these areas would have been excellent places (in the writing of the adventure) to talk about interesting skill challenges and give good examples of how to run them.