I am tempted to say that one reasons is that the folks see this as income during the recession - grab it whilst it's there but actually what I want to say is ...
Perhaps it is like when a parent inadvertently lets a child run into danger?
They pull the child back from the road/danger and scold them loudly. Actually, they are scolding themselves for the fright they had that they allowed the child into danger. They cannot sue the child or themselves but they can sue the airline.
If there was a cooling off period of 364 days, so that could asses what injury (mental and physical) they have and if they really ARE pleased to be alive ... perhaps fewer people would sue?