Because Wisdom has been traditionally personified as female, courtesy of the Greeks and other cultures.
Proverbs predates the Greek invasion of the middle east by over 600 years, it's more likely that the Greeks got it from reading Solomon's proverbs.
Consider Proverbs for what it is: a poem, it's important to keep that in mind as you read. If you read Treasure Island and ignore the context you may mistake it for a historical document, but it's a novel, a tale of imagination.
Proverbs 8 is a specific type of poem called an encomium—a poem of praise. So, when we read that Wisdom is a “she,” understand that
Proverbs is heavily artistic; therefore, we are not reading a technical definition of wisdom.
Also: English does not use grammatical gender (classifying words as masculine, feminine, or neuter). However, the Hebrew language (in which Proverbs was written) does use grammatical gender, much like Spanish, French, and many other languages do. Herein is our problem. “She,” as we understand it, is not necessarily “she” as it was intended in Hebrew. In English, the word
wisdom is grammatically neuter, but not so in Hebrew. The Hebrew word is
chokmoth, and it is grammatically feminine. In Hebrew, it would have been natural to speak of wisdom as a “she.”