There are a couple of possibilities. One would be that this particular part of the pond is free from predators and is basically a safe area with good food, shelter, bathing, preening and resting resources.

Another possibility is that there is a nest in the area and the bird is sitting nearby the female and nest. Are there any reeds or high vegetation in the area where the swan is sitting?

Finally, and you make think we are joking, but from a distance we have been fooled, in that someone has put a swan decoy on the pond. People use fake swans, geese and even ducks to adorn an empty waterway or hope that they can attract various birds to the area. In other areas, people place the large bird decoys on a pond, thinking that they may repel certain species as Canada geese, etc.

If this is not fake, there could be something wrong with the swan such as injury or entanglement. If possible, we would suggest a closer look to see that there is nothing wrong. If there is, then the swan needs to be reported to a wildlife rehab facility fir capture and treatment. The Regal Swan