Eli can't do the job alone. Philip can. It's as simple as that.
Philips can???
2004--Rivers received very limited playing time in the 2004 season.
2005--Backup QB
2006--Rivers became a contender in week 5. But he couldn't get the job done this year.
Rivers led several 4th quarter comebacks in 2006, and posted the league's highest 4th quarter quarterback rating. His performance over the season led to a selection to the 2007 Pro Bowl. After a 14–2 season the Chargers were set to play the New England Patriots in the divisional round of the playoffs. The Patriots won 24–21.
2007--Again Rivers couldn't get the job done
After a 1–3 start, the Chargers found their groove finishing 11–5 and winning the AFC West for the second straight year. The Chargers also won their first two playoff games since the 1994 season, beating the Tennessee Titans and the Indianapolis Colts before falling to the Patriots in the AFC Championship Game 21–12.
2008--AGAIN Rivers couldn't get the job done
Fortunately for Rivers the AFC West were quite poor and they were able to win the division over Denver by a tie-breaker. On January 3, 2009, the San Diego Chargers defeated the Indianapolis Colts 23–17 to advance to the AFC Divisional Playoff. Rivers led the Chargers down the field down 3 points with under 2 minutes left. He set up Nate Kaeding for a game-tying field goal that would lead to overtime, where the Chargers would go on to win 23–17, thanks to a Darren Sproles touchdown. In the divisional round versus the Pittsburgh Steelers, Rivers played well throwing for over 300 yards and 3 touchdowns but was sacked 4 times and threw an interception. Despite his efforts, the team fell short and lost to the Steelers 35–24. They possessed the ball for only 17 seconds during the third quarter and had only 15 yards rushing.