It's random like anything else. My temporary noob camp was near a river that had a swamp at the end of it. That was the first swamp I came across after walking for hours. While exploring I've also come across areas that have more than one swamp within a short distance of each other. I've found them landlocked, near rivers, and near lakes.
They're out there. Might be less common than they used to be, but they can definitely get bigger.
"The psychological trials of dwellers in the last times will be equal to the physical trials of the martyrs. In order to face these trials we must be living in a different world."