Isn't the whole point here that handshakes are cheaper compared to the actual content? If you use the content as the handshake itself, you can end up with huge content only to find out that the conversation didn't work.

That conversation can also go like;

A: Can you hear me?

B: Yes

A: What is the full unabridged story of War and Peace?

B: <...>

A: I am going to a take a nap for now, goodbye!

B: ... Goodbye!