In situations where carriers know in advance that oversale is close to a certainty, calling passengers who are likely to volunteer makes sense. Those passengers might well be quite happy to stay put and have their hotel & meals covered and not have to go through the nuisance of checking out, heading to the airport and then being rebooked.

For the most part, overbookings do not result in oversales. No shows via misconnect, bad traffic, illness, and generous bartenders will often and are predicted to deal with the issue.