Why do I need to connect a Facebook page?

Instagram requires connecting professional (business/creator) accounts to a Facebook page in order to be accessible through the Instagram API (which 3rd party apps need to operate).

See this quote from Instagram's documentation:

"Instagram Professional accounts must be connected to a Facebook Page before their data can be accessed through the API. Once connected, any Facebook User who is able to perform Tasks on that Page can grant your app an access token, which can then be used in API requests."

Source: https://developers.facebook.com/docs/instagram-api/overview#pages