A more obvious setup to the joke is

"Is there a doctor on this plane???"

"Well yes, but I defended my thesis in comparative literature..."

Ugh. Of course. That makes a lot of sense.

Didn't come anywhere close to getting it on my own though!

Right, but what makes it work for me is that you have to bring the setup to the table. I think The Far Side is similar in a lot of its humor.