Because it’s absolutely exhilarating to throw stuff into the water (or off cliffs, or into holes, etc). Not condoning the behavior but you gotta admit it’s pretty fun.
When I was in high school I worked at a supermarket, and one of the duties all the male high schoolers had was to round up the shopping carts twice per shift.
The supermarket was built on landfill, and parking lot was a slope with a cliff at the end. Any shopping carts with wonky wheels were "accidentally" set free to roll downhill, slowly getting faster, until the front wheels hit the curb with such speed that the entire cart flipped over and tumbled down into the ravine.
The ravine has since been filled in and turned into a movie theater. I wonder if there are hundreds of dead shopping carts entombed underneath.
We found a car2go in the river once, apparently.