while most nymphs reside in secluded areas like forest and lakes some of them decide to move on to human settlements.
The obvious reason would be that humans would rarely pass through places that are known to be 'haunted' and nymph would easily get tired of copulating with the same forest denizens and animals over and over again.
Aside from this, nymphs consider human semen as an irresistable delicacy and they crave this more than anything else.