There's enough evidence that plastics leak harmful chemicals into our food that I don't want to eat anything that's been sitting in a plastic bag in warm water for hours on end. No thanks.
Sous-vide isn't that prevalent in the mass-market chains, but what is used is the technique of preparing the bulk ingredients in a central kitchen, vaccuum sealing in plastic and then transporting to the stores.
The stores then rewarm the food in a hot water bath before serving. This technique enabled a chain like Taco Bell to drop all cooking in-store. It also raised their average sanitation scores since the risk of undercooked food is practically nil.
Even the beloved Chipotle prepares some stuff offsite, like the barbacoa and carnitas.