A couple of things -

1. The headphones may be noise-cancelling, but they certainly don't cut out the sound or feeling of a plane crash landing. I certainly hope you would notice crash with or without headphones on.

2. How else are the crew supposed to re-collect the headsets? They recycle them for use on further flights, and collecting them 20 mins before landing definitely ensures they can recycle more then simply asking people to leave them on their seats prior to disembarking, and having to deal with picking them up then.