They could have. But every moves needs to be judged and weighed . How much will the contract drag the team down when Harper is 37 and can't play? If the young players from the rebuild do well, the team may not have the resources to get additions because of that contract .
I'm not saying they shouldn't have signed one of them, they should have. However, there are some valid reasons why they may not be a wise use of resources for a team that historically doesn't have a high payroll from a FO perspective.