Actually, I am surprised more stealing is not around. I am always amazed by what guests leave in strollers, at benches/seats, tables, on the ground while they ride rides, stand in line, have photo shoots with characters, etc. And more often than not they are no where in view or access to their items.

But considering many don't keep their kids in sight, I guess not having puchases or a camera in sight is no surprise.
I would never think about going to a mall and leaving expensive and/or personal items in a hall and expect them to remain untouched if I was away from them.
An example was earlier this year we were in Fountainview Bakery (Epcot) having breakfast. A mother father and kids left everything they had (full backpack, bags, etc) at a table while they were with characters in another part of the building. And they were gone quite awhile. No one touched their items while we were there. But certainly there was plenty of opportunity for anyone.

The father asked the CMs behind the counter to watch their stuff. They have customers, with many coming and going. I doubt their attention is focused on watching one guest's items that was enough stuff for a pack mule, and for several minutes. This guest had so much, I don't know how they kept track of it.
Now the thieves who are brazen are those who take items right underneath guest's nose while theor head is turned and focused elsewhere. And even that happens.
Disney is like a city. And everyone (Cast and Guests) should use common sense and pay attention to their surroundings.