From where I sit, the reason for this is that the Sox FO have wanted the young armsto leap forward and stick in the rotation. These "band aid" types (Richards, Wacha, Hill, Kluber, Paxton to some extent) are much easier to jettison than a guy on a long contract.
I'm not arguing that it's been wholly successful, but this strategy has worked to some extent. They've clearly been invested in Whitlock and Houck sticking in the rotation, and there have been some successes there. Bello and Crawford's development have been really positive things for the org, and I'd argue that one or both might not have had the same opportunity to develop at the major league level had we signed a guy like Stroman two years ago, or Eovaldi last year.
The way injuries shaked out, sure, of course those arms would have helped. And it's complicated matters that we've had to reserve a spot in the rotation for Sale that he's seldom used. But in terms of strategy, I think they have prioritized getting MLB starts for Bello, Crawford, Houck and Whitlock.