Several good reasons have been stated so far. I would include lack of imagination as a reason. Some fans or media wish to create or improve upon a team nickname, but do not have the innate creativity of doing so. Thus, they take the easy road and try justifying how the phrase fits their team better than Dallas. It's certainly not an unusual practice. It's the main reason why so much stuff gets regurgitated by Hollywood all the time. One day, someone will come up with an original nickname that matches a team's personality and success. Some of those always wanting to steal America's Team for their purpose will stop, consider, and say, "Huh. Why didn't I think of that?"
Hopefully, someone else will overhear the comment, turn to that person, and say, "It's because you are an idiot." lol.