If a random unauthenticated user can find it, which is what I am assuming people mean by making it accessible instead of locked in a discord, then other companies can pull and use the data too. What benefit does discord have to gain by making their content more accessible, that outweighs the possible losses?

Discord having the ability to search through their history and provide knowledge is not the same thing as allowing unauthenticated/anonymous users to mirror all the data. The limiting factor is a human responding to the same question 1000 times or bad search through a channels history. Although one solution would be to give all their data to Google and hurt themselves, the alternative would be to make their own search better within their ecosystem.