Why doesn’t Blizzard like to reuse areas?

We have all of the awesome cities from prior expansions and they are all empty. To Me this is a huge bummer. I don’t understand why we get a completely new city most of the time instead of updating and reusing old assets. It would be fun to have story lines lead back to old areas that were updated. We have this huge amazing game and most of it isn’t used because we’re all in new areas.Edit: I saw a reply talking about how the major cities etc could be utilized better and I agree with that completely. I hate that players always want new areas and content when we could have updated older areas that serve new purposes. No one goes on vacation to some place they love and then never returns (excluding financial issues etc), people go to Disneyworld and land over and over again because they love it not because it’s always new.