As a current fellow, I am now bridging the period between resident and attending, and I can see both perspectives.
For example, I am final contract negotiations for my position next year, and I have been for over a month so far. I am dying to tell people where I am going, but I have to keep things mum in case things fall through.
I cannot speak for the situation for any particular attending, but I can understand why they would want to keep mum until they have fully closed a deal with their future employer.
Similarly, people often feel a sense of loyalty and indebtedness to their prior institution. For example, I currently work for RIC, but did my residency at Kessler, and feel a tremendous sense of loyalty to both programs. So, whenever possible, I try to extol the virtues of both institutions. It can be difficult to do this and at the same time provide full disclosure to residents or prospective residents. Its a delicate balance.
In fairness to residency programs, there is information asymmetry on both sides of the residency selection process. For example, many applicants may have "research in progress" or something similar on their CV, and thus have been highly ranked because of their research interest. Most medical students won't fully disclose that they have stopped working on a research project, or that they ended up not covering an event listed on their CV, etc. So both sides are not presenting a full picture.