That's half your starting rotation last year.
Brown pitched 13 games. That's about a third of what an active starter would pitch. Randy Johnson also had a better year last year than this, though only by 3 losses and 1.3 points in his ERA. You're stacking the Johnson of 2006 with the Kevin Brown of 2005 and saying that the starters suck.

We are talking about pitching here, not scoring runs.
Think about the correlation between ERA and wins. Check out his year off the DL with the Diamondbacks: 16-14 out of 35 starts. Shitty year, right? The guy had a 2.6 ERA. His team was not scoring runs for him. Even with his ridiculously low ERA, he wound up averaging 7 innings on the mound each time he came out. Now he's packing a 5 ERA and still walks off with 17 wins, 2 less than the pitcher with the HIGHEST amount of wins this season... who is also on the Yankees.

And the reason it matters to look at post-season and second half of the season is because that's when these ancient pitchers really show their age. They can't put together a full season.
The entire professional baseball establishment is hurting for pitching, or else any not-sucking-too-much left hander with a halfassed curveball would not be fetching multi-million 3+ year contracts. On top of that, most teams do not end the year with the same active pitching staff that they start with.

If you do fine during the regular season (which they did) and the wheels just come off in the postseason (which they do), it has something to do with the management aspect of the ballgame. Provided, of course, you want to pass on the whole "The Yankees can't handle postseason pressure" argument, which I assume almost anyone will. You've got a better chance of just saying "well, it's just the randomness inherent in the game". When a guy like Jeremy Bonderman has an average year with average stats literally across the board and goes out and pitches a perfect game into 7 innings against the Yankees lineup in the postseason while the Tigers just beat the whole club up (with Mussina starting), someone is making the talent do something wrong.