ELI5: How is lettuce able to be picked, shipped, processed, and shipped again, and still be green but if I don't finish a bag of premade salad in a week the lettuce turns brown?

Holding lettuce in water during processing and keeping them stored in a cool, low oxygen environment helps keep them fresh from picking to processing.
Once processed they are packed in air-tight sealed containers with a mix of gasses aimed at keeping their shelf-life and nice green colour.

Generally if you want to keep your salad pretty green store it iced-water in the fridge.

Explain Like I`m Five: good questions, best answers.