Why do we assume the government should run airline security?

Why do we assume that government has the answers in terms of best safety practices for airlines? The airplanes passengers fly on are the property of the airlines. Each airline should be free to determine whatever security practices it believes is appropriate for the safety of the airline and the passengers. Airlines have every incentive to focus on protecting the airplane: not only is an airplane expensive to replace, but also, the loss of good will from an incident could easily put the airline out of business. Secondly, by having a diverse number of security practices, it would create a lot more unique solutions to securing the airline. Thirdly, airlines would never impose policies that would result in passengers being harassed. I highly doubt airlines would, if they could, pat down elderly grandmothers, children in wheelchairs, and toddlers, but the government does so, because government is about force, not wisdom.