Well, I can't speak for the other airlines, but AA's gate lease at DAL specifically states that the area can be used only for office space. There aren't even jet bridges attached. And, there is the little issue of available "working" gates at DAL. SWA has 14 of 16. At DFW, almost ALL of Terminal D is available, and some of Terminals B & D. AA has exclusive use of only A & C.
And, don't give me the "Master Plan" argument of 32 gates at DAL. They don't exist today. And, given Dallas's success with Master Plans, don't count on them being there any time soon. Look at the Master Plan for the Trinity River development. Lots of paper, lots of talk, but when it comes to someone or some organization, public or private, actually putting up some money and getting started on the work, it ain't happenin'. There's still just a very small stream that sometimes floods to a mile wide and a foot deep within the levees.
In Dallas they do things like grant tax abatements to Hunt Oil to build a new office building downtown while downtown Dallas has the highest office vacancy rate of any metropolitan area in the U.S. (in excess of 20% of available office space is vacant). They should be granting tax abatements to companies willing to move into existing office space instead of constructing more buildings in the suburbs which benefits Dallas only in the "whole metropolitan area economic strength" sense.