When waiting to board a plane I would much rather wait in the terminal than on the jetway. It's nice to have a seat, laptop, internet, power, and a pleasant temperature. As ticket scanning has gotten faster, however, this has meant more time on the jetway and less in the terminal. Here's a simple model of plane boarding:

• Waiting in the terminal (pleasant: 8)
• Waiting in line to have your ticket scanned (unpleasant: 5)
• Waiting in the jetway to board the plane (very unpleasant: 2)
• Waiting in the plane's aisle to get to your seat (unpleasant: 4)
• Waiting in your seat for the plane to leave (pleasant: 7)

The key problem here is that the limiting factor is people getting into their seats. As long as people are always getting into their seats as fast as possible there's nothing you can do to speed up the process. Having a short line on the plane to keep boarding running at full speed is good, but quickly getting everyone through ticket scanning only to have them wait on the jetway makes things worse to no gain.

If people were let onto the jetway only when the queue on the plane got too short, then you'd minimize waiting in unpleasant stages.