For at least one of the three legacies (wouldn't surprise me if the other two are following suit for the same reasons), the slow in hiring is to allow their training department to catch up. It's a waste of time and resources to offer applicants class dates 5 months out, only to have said applicant receive a job offer from another carrier with an earlier class date.

It's easier to tell the share-holders the current headwinds are from unexpected outside factors, rather than "We put ourselves in this position with early retirement buyouts that were too aggressive". I think Boeing is the convenient scapegoat that showed up at the "right time".