Here's why:
* Historically: "Swain" is an old word for a young man, often associated with a shepherd or a lover. Its feminine counterpart, "swainess," was used to refer to a young woman, especially one with a similar rural or romantic association.
* Modern Usage: The word "swainess" is practically absent from contemporary language. We generally use other words to describe young women, such as "maiden," "damsel," or simply "woman."
Therefore, while "swainess" technically exists, it's not a commonly used word in modern English.