He is getting double teamed and chipped by a TE or running back damn near every play. This happens because the rest of the defensive line is being handled one one one. Nothing much Hardy can do about that, no one is good enough to consistently get pressure on the QB having to go through 700 to 1000 pounds of man damn near every play. If Crawford or whoever plays DT beside him starts playing better then Hardy will go back to being the guy he was in the first half of that Patriots game and who he was in that Seahawks game. That is the nature of the defensive end position, you need a DT to at least suck up some blockers or else you can't really be all that effective. Also dude had not played football in over a year. I would expect him to take some time to get into game shape. So if the DT's never start playing any better then yeah Hardy is just going to be another end for use who shows up sometimes and not others. Same for Lawrence. Same for Gregory. Same for every defensive end in football.
As far as the him missing meetings and stuff, I would expect the coaching staff to take care of that. It's pretty obvious to me that Garrett if not outright scared of Hardy doesn't really want anything to do with him. Hardy get's into it with teammates and coaches Garrett says he saw it but decided not to do anything. Hardy makes a post taking a shot at the media who have done nothing but take shots at him since he became a Cowboy, Garrett "has a talk with him." That is pretty inconsistent in dealing with a guy who is pretty out there like Hardy is. And I certainly don't believe that Hardy is the only player on this team to miss a meeting this year, but somehow only his name came up.
I assume he stays here because quite frankly no other team in the NFL will take him. Nobody wants the media show. Nobody wants to have to answer questions about him even when he did nothing of note after every game like Garrett has to. Nobody wants to have to deal with him when he does one little thing wrong, like miss a meeting for whatever reason.