Some of these places have a deal where they make the first-years commit to a fellowship with the implicit threat that a refusal to do so will hamper their evaluations. So in addition to plain old lethargy, the fact that residents stay at the mothership for their fellowship can have more sinister reasons (IR fellowships at big-name institutions have remained unfilled in recent years, some of the more desireable fellowships are actually in smaller community type settings. programs will do a lot to make their fellowships look good by being filled internally every year).
