It's just too risky.
And with Orlando coveting young talent, high draft picks and cap relief, the Thunder isn't an ideal partner to begin with. Oklahoma City's draft picks would be in the late 20s, and the Thunder can't afford to take on massive contracts like those owed to Hedo Turkoglu and Jason Richardson.
From a financial standpoint, Oklahoma City also wouldn't be any better trading for Howard. If the Thunder must eventually trade Harden or Ibaka for monetary reasons, acquiring Howard would not be the deal that provides relief. It'd be a lateral money move, perhaps even having a negative effect by the time the rest of the roster is filled out.
You also can't overlook the fit.
Howard is a player who has gotten a coach and a general manager fired, made a mockery of the Magic organization with his demands and is growing an increasingly negative reputation for not being about winning. The Thunder runs from players with that much baggage. Adding Howard's ego to a team that already must be careful with how it massages Durant and Westbrook's personalities would be like playing with fire.
But on paper a trio of Westbrook, Durant and Howard instantly would become the best in basketball. It's what makes the speculation so scintillating — even if Howard wants to play for the Thunder about as much as he wants to play for the Bobcats.