Because it's still an advantage. They are relieved of the burdens of mismanagement. While they won't win every year, their money makes them competitive regardless of how stupid their decisions are. With the possible exception of teams in LA, no one else can do this. In addition, it distorts the market. Other teams can plan smart, but they can't save for the big final piece because they'll be outbid by NY for him. That's the problem.
For every other team, the luxury tax is enough to remedy this without having to go to the harsh inflexibility of a salary cap. But the Yankees are apparently immune to this as well now.