Insurance companies don't directly pay for the total costs of pre-approval so they optimize to minimize personal costs independent from the total cost to the system.
In other words doctor's offices don't bill them for the time a sectary spends filling out their forms. Which means doctors include that overhead cost as part of the cost of procedures. Also while each insurance company's internal system is streamlined each doctor's office needs to deal with several companies’ which increases their overhead costs even more.
It's like a large game of Prisoner's dilemma where several people have already defected. http://en.wikipedia.org/wiki/Prisoners_dilemma
PS: A doctor making 200k/year spending 5 minutes on a pre aproval cost's someone more than 10$. The patent needs to show up which costs them even more money etc.
