A lot of valet workers consider themselves to be unfairly paid. This is how some of them justify stealing from their customers' cars. They feel they DESERVE IT.
Others steal because they simply believe they can get away with it. And it is a way to make extra cash.
Even if they are trustworthy, it's always possible they will forget to roll-up a window or ensure that all doors are locked. And then a passerby finds that an opportunity to plunder exists.
The video in this article catches a lying, stealing valet in the act, and then the confrontation with a news reporter that follows...