[paxys]

Because they may change their plans. They could be a no-show (which will affect your return flight). They could call and change the flight without your knowledge. They could add extras to the trip and charge it to your card.

People are flaky, and being on the same itinerary with the same PNR as someone else means your trip is in their hands.

[w29UiIm2Xz]

Some sort of service that sat on top of bookings would have its own set of terms and conditions that you agree to, which would at least disincentivize them from acting against your interest.