Because they should have known Giolito and Pivetta would go down?
Even with Giolito out, the starting 5 opened with a fantastic run, completely justifying their use in that role.
Give it a rest man.
I'll give the criticism of ownership's unreasonable spending limits a rest when the plan as presented shows itself to have worked out, or when you stop popping off on anyone who happens to hold an opinion contrary to ownership's desire to increase their profit line at the expense of the team's competitiveness. Giolitto was already down and the Sox FO had the rare opportunity to sign a very good starting pitcher who was still available, and they choose not to bolster a thin starting staff.
If you think that the small sample of the season so far somehow proves that their plan is working, then don't you also have to accept the trends indicated by that small sample? The team had one good turn through the rotation, and the next turn showed diminishing effectiveness; how is that not indicative of the plan not being able to hold up? The complaint has always been that ownership set spending limits that prevented the team from being improved, and the lack of depth is telling.
Just curious, where do they keep this pitching depth that they should have built up more? Let's say they sign at least one more starter in addition to Giolito (Montgomery, Snell, whoever). That pushes one of the guys who is in the rotation now (Houck/Whitlock/Crawford) into the pen? Does that guy still count as rotation depth if he's a reliever? There's only so much depth a team can have such that they can survive severe injuries to two of their starters. The teams that have that kind of depth tend to be the ones who develop pitchers themselves, not shop for them on the open market. In other words, the "fix" for surviving Giolito/Pivetta injuries isn't really solved in the off-season.
In this case the Sox did have the chance to fix this problem. Monty was available when Giolitto went down and it's safe to say they likely could've added him to fill the same roll they intended for Giolitto. It was a unique opportunity... the way things are going he'll probably blow up too (I've already thrown my tin foil hat in the right about his medicals).
I'm not arguing that those staffs you present wouldn't have been more successful. I'm arguing that it's not a means of building depth. You don't build depth through free agency. You build it through developing pitchers. You can't do that if you leave no room on the roster for them (or rely on injuries to clear the way for them).
In fairness to
@Midnight Ryder Jones, any team probably needs to do both. On the Sox litterally none of the rotation signed here as free agents, and 3/5 of them are young guys who could've wound up in the bullpen and it wouldn't have been a huge injustice, especially after last year. Certainly there would be some dissent and we'd all think that someone else should be in the rotation and someone else demoted, but if B&B had brought in another veteran arm to take the workload, would anyone really have batted an eye?
The other thing to keep in mind is that finding players whose potential you can unlock isn't limited to minor leaguers with lesser pedigrees. I feel like the "deep depth" lessons of the 2013 squad are getting short shrift, and if injuries are going to keep piling up across MLB and the Boston roster, it's not going to be an underappreciated part of the game for very long. Unfortuanately, the two worst areas for the Sox (SS and SP) is where they got hit to start.