Who sets the meeting point?

It's the partner: the pilot for a flight, the owner for a rental. They alone know their airfield, so they're the one who picks the exact spot where you'll meet.

Why not the passenger

The passenger or renter can't change the meeting point. It's a safeguard: you don't move a meeting point you don't control. If the spot isn't clear to you, just ask for details directly in the conversation.

On a closed booking

Once the booking is completed or declined, the meeting point can no longer be changed, just like the chat.