Because it isn't. From the New York Times:
The United States does not have a law requiring the repair of used vehicles — including rental cars — that have been recalled for safety issues before they are rented or sold to the public. Used-car dealers and rental car companies are allowed to fix problems when — and if — they see fit. And they are not required by law to disclose to customers that a vehicle is the subject of a recall.
Even as Congress and law enforcement officials investigate the delayed recall of 2.6 million General Motors cars for a faulty ignition, auto dealers and many manufacturers oppose efforts to require recalled used and rental cars to be immediately repaired. These efforts include a measure recently sent to Congress and a separate Senate bill that has languished since 2011.
(A group of "major rental companies" agreed in 2012 to fix recalled cars before renting them, but they're under no legal obligation to do so, and the process isn't subject to regulatory oversight.)
Have a pleasant weekend out there on the roads.