It'd be too awkward

In his article, Aaron says that Punk would make a fortune if he returned. This is hard to disagree with, if the two were able to sign an agreement, the WWE would have to pay the big bucks to Punk.
But the second Punk would step on to the ramp, we'd all remember. We'd remember every negative word shared between Punk and the company. We'd remember the libel lawsuit and the subsequent lawsuit between Punk and long-time former friend Colt Cabana.
Cody Rhodes' uncle is a WWE Hall of Famer. More details HERE.
In his initial run, Punk spoke about how much he loved wrestling and did so for the fans. If the only reason he returned was for a payday, well, every single fan would be able to see his heart wouldn't be in it.