The short answer is simple: they don't have a true number one pitcher. The long answer only gets worse.
Cole Hamels will never be a number one in the mold of, say, a Steve Carlton. Opposing batters don't start worrying about facing him at 2PM the day of a game. Last season's over-exertions may be part of the explanation for Hamels' mediocre performances this year but the larger issue may be he is a two-pitch pitcher and only one of those pitches, his change-up, is outstanding.
The rest of the Phillies staff is hardly the stuff of legends...as we've all said ad nauseum. Some day, and it will come soon, Jamie Moyer is going to consistently show his age. Joe Blanton is a middle of the rotation guy. J.A. Happ looks to have a good future but he's a finesse pitcher at a relatively tender age and those types of guys are never the ones who lead a staff.
Roy Halladay is a number one pitcher by all accounts. If they can get him, the Phillies must act.