I did. Look at the picture provided it's the authenticate step that's the problem. Alice wants to send Bob a message. Alice contacts public key authority and says give me bob@bob.com's private key, encrypts the message and sends it. So far so good.

Now, someone contacts the key authority and says I am Bob@Bob.com what's my private key. Without prior communication between Bob and the key authority there is no way to do that exchange over an open channel securely. Assuming email addresses are public information and someone can get bob's email address before he communicates with the key authority.