Because with airline seats (and cruise cabins, too) everyone has the same arrival and departure days. With a timeshare system like
DVC, that's not the case.
Imagine a hotel with just two rooms.
Guest #1 wants to stay Sunday to Tuesday.
Guest #2 wants to stay Wednesday to Saturday.
Guest #3 wants to stay Sunday to Saturday.
If the hotel is assigning rooms, obviously they put Guests 1 and 2 in the same room (#2 takes the room after #1 departs.) Guest #3 would get the second room.
But if you let them choose, what happens if Guest 1 chooses one room and Guest 2 chooses the other room? Now you've got two rooms that are open just a few days here and there instead of being able to accommodate someone wanting a full week.
That's an oversimplification, but it would happen on a much larger scale if we were allowed to pick our own rooms. Scattered all over the resort would be rooms sitting empty for 1-2-3 days because of the schedules of arriving and departing guests who have selected the room.
I think you'd also see the birth of a new form of rental business. People would quickly identify the most desired rooms at each resort (Epcot view at BCV, close to elevators at BWV, odd-shaped rooms with extra large balconies, etc.), book them exactly 11 months out, and start auctioning them off to the highest bidder.