One Man's Trash?

With training camps just one month away, NFL teams are hustling to fill out their rosters. It strikes me there are some pretty accomplished players still out there, without a team, as camp approaches. Especially when you look at the quarterback ranks, it seems like some players who should be at or near the peak of their careers, are instead on the street right now, waiting for a call.
The best example is Daunte Culpepper (above), a veteran quarterback who has a strong resume. He's a free agent, with 90 NFL starts in his background and he's only 31-years old. You mean to tell me Daunte Culpepper can't help an NFL team get better at the QB position?
What about Byron Leftwitch? He's only 28-years old, and he's started 46 NFL games. Former Bills starter Kelly Holcomb is out on the street right now, with 24 starts in the league. Even Tim Rattay figures to get somebody's interest. He's only 31-years old and he has 18 starts in his NFL resume.
It's not just names that are out there, unsigned with one month to go until training camp. There are some proven performers. I know NFL teams like to wait until training camp to see where the injuries hit, before they commmit to veteran players. But why not take a flyer on a proven veteran; sign him to a one-year deal with back-loaded incentives, and grab up an accomplished players while he's still available?
What am I missing? Why do you think some of these proven NFL players are without teams right now?