Why is cat kidney disease common? Does wet food or water help?

Kidney disease in cats, in most cases, is caused simply by a loss of function overtime. I think we see this more in cats because they tend to live longer than dogs, sometimes on the upward end of 15 to 20 years. As the pet ages, so do the cells that make up the kidneys, and this results in a loss of function (kidney insufficiency). In theory, yes we can preserve kidney function by adding more water to their diet. But I think that at some point the kidneys will still lose function because that is what happens naturally as the pet ages. With all that said, I still encourage owners to feed their cats wet food and use kitty water fountains whenever possible, to preserve kidney function for as long as physically possible.