One answer TSA came up with was "because private screeners are 17% more expensive than TSA screeners." Turns out that was not quite true, and it wasn't true because TSA made it up:
The Government Accountability Office (GAO) said TSA’s new estimates show that private screeners are just 3 percent more expensive than federal workers - not 17 percent, as the agency previously had stated. Auditors said that earlier TSA estimates had not accounted for the costs of workers compensation, liability insurance, retirement benefits and administrative overhead involved in using federal employees...
Mr. Mica said the revised 3 percent cost difference is likely still too high because it does not take into account "the full cost of TSA’s bloated and unnecessary bureaucratic overhead."
TSA officials are developing a pretty bad habit of fibbing to the public to cover their own bureaucratic hides. Whether it's covering up the damage from their security screw-ups or denying that full-body scanners can save images or being argumentatively dishonest about unionization, it increasingly seems like they'll say and do anything to undercut airline security debates.
It's not exactly what you want from an organization supposed to restore the public's trust in flying, but at least they're also asking us to pay more airline fees to them.