Because you'd need to look at recent records. The wins against Nole were slanted earlier in the rivalry, not recently. Del will probably beat Andy, if he doesn't Nole is likely to blow him off the court again as he did at the Olympics. One has to give things more thought then just looking at pure numbers without the history behind them.
Maybe you should have looked at the head to head in more depth before lecturing others on it?
Peak Roddick was 03-06, Djokovic won a slam in 2008 and "arrived", he didn't get any better until 2011 but he appears to have fallen to his previous (2008-2010) level.
At the end of 2008 the head to head read "Roddick 1-2 Djokovic"
In 2009 and 2010 the head to head was "Roddick 4-1 Djokovic".
In 2012 when Roddick is in terminal decline and apparently injured it is "Roddick 0-1 Djokovic".
So actually Roddick has beat current level Novak plenty times, thrice in a masters series and once in a grand slam.
Clearly the Roddick from Wimbledon/Olympics will not have a chance v Djokovic, but if he ups his game to its pre 2011 level seeing as it's his last tournament, home crowd, whatever, there's nothing to stop him winning.
Personally I think he'll lose to Del Potro in 3, but it is entirely true if he beats DP he can beat Djokovic.
Sometimes you just need to give these things more thought thAn just looking at pure numbers without the history behind them.