It's not because of any insidious details of the contracts. It's an unusual tidbit of copyright law that a work of the government is not copyrightable. But work done by contractors is not done by the government. It is done by a private entity that is perfectly able to hold and retain copyright. The incentives are such that the government doesn't really care whether it (or the people) have ownership of the IP rights, just that the government is free to use it. So the contractors sell the government the unrestricted right to use it, for government purposes only. The contractors often offer a smaller bid for letting them keep the copyrights, which from a direct, first-order financial perspective is a better deal for taxpayers.