Because the encryption keys are generated in client browser and never sent to the server.

All this encryption takes place at client side, so that even the server administrators have no access to the data.

But creating an account can be useful, too: while having an account the key exchange can be done through public key cryptography.

It does not save any cookies or have any external links (like Google Adwords for example) that could track you.

The source code is available here. Being open source allows anyone to audit the code.