Because they're being smart. Sure they could be like the Marlins and Cubs and leave their guys in until their arms fall off. But look at the Marlins: Burnett out for the year, manager fired due to keeping pitchers in too long. Kerry Wood gets TJ surgery, Prior might be out for the year.
I can't blame Oakland for any decision they make with their pitchers, since Zito, Hudson, and Mulder's arms have been healthy due to using them properly.
"Jack, will you call me, if you're able?"
"I've got your phone number written, in the back of my Bible."