If there is already one plane full of late travellers who might need to be rebooked or put up overnight, it just doesn't make financial sense to do that to another plane full of people just out of "fairness". It's a very disruptive and expensive activity, and you also have to consider that downstream connections for the 2nd airplane will also be affected unless there is an extra plane and crew available.

Sometimes it's a good thing to have your flight delayed: On Sunday afternoon/evening everything was booked solid for DTW-ORD/MKE/IND-MSP and we were unable to confirm any standby for DTW-MSP, so my wife & I went on unconfirmed standby for a flight that ended up leaving 90 minutes late. At first I was annoyed that we had been put on standby for the one flight to MSP that got delayed but quickly realized that it likely would work to our advantage if a couple pax bolted during the 90-minute delay. We were to first two (and I believe the only two) standbys to clear.