Why is it that web developers do not do this with databases?

I hear about all these websites getting hacked with sql injections and stuff. What's preventing them from encrypting the hashes with a 32 character string? If I were a hacker and I managed to get the database and I came across encrypted hashes I would not be able to do anything with the database as I do not know the encryption algorithm and the key.

As long as the key being stored securly everyones account would be safe.