Grammatically, the association makes perfect sense. The Latin word for 'man' is vir. Virtus, or 'manliness,' is formed by adding the abstract nominal ending -tus. In Latin, as in German and modern Romance languages, most abstract nouns are grammatically feminine. Nouns ending with the suffix -tus are no exception: virtus, iuventus, senectus, etc. all take feminine adjectives. Since virtus is grammatically feminine, it is unsurprising that its personification would be female.

We see something similar with the personification of the virtues in the Middle Ages. Even though women were often assumed to be less disposed to virtue than men, virtues like faith, hope, charity, and humility (all grammatically feminine in Latin) were represented allegorically as women.