The short answer is: Because they can't score anyway. But that's not entirely true. The A's can score a run if you give them 5 innings a lot better than they can score a run if you give them 3 innings, in the same way that a .200 hitter (like Kouzmanoff!) can get as many hits as a .333 hitter -- if you just give him a lot more tries. (Sure, Ichiro can go 5 for 15, but Kouz can get 5 hits too if you can just get him 25 at bats.)
Going back to the decision to pinch hit for Trevor Cahill on Friday, the move was made to try to score a run in the 7th inning and scoring runs seems like a great goal for this team right now. However, Cahill not only had a one-hit shutout through 6 innings, he was getting better and better and he was at 83 pitches, so he likely could have thrown 2 more innings.
Had the A's punted the top of the 7th, in exchange for two more (hopefully) scoreless innings from Cahill, the main impact would have been that the subsequent scoreless innings from Grant Balfour, Michael Wuertz, and Brad Ziegler, thrown in the 7th, 8th, and 9th, would have been tossed instead in the 9th, 10th, and 11th, giving Oakland's hitters at least five more turns at the plate (8th-12th) to try to take that elusive lead. As icing on the cake, in the 11th and 12th, and any innings beyond, Oakland would have found itself batting against the likes of Dan Runzler and Guillermo Mota, the only relievers left in the Giants bullpen at that point.
But the point is this: Since the A's have no way to jumpstart the offense by putting in better hitters or manufacturing runs in some superior way than the way they are trying to do it now, their best chance is to give the offense "five innings instead of three" (actual numbers may vary depending on the actual game), which means putting your best pitchers out there for as long as possible -- because good pitchers are something the A's actually have.
In other words, these A's, currently, can't score but they can prevent like the dickens, so prevent as long as you can because the longer these guys "hit" the more inevitably they will accidentally, finally, eventually...score a run.
