It isn't about cat pictures, it's about sending a message.
The message we need to send is that Internet traffic is private and encrypted by default. You don't "turn on" encryption only when you're doing something which needs to be hidden any more than you only use envelopes when you're mailing something which needs to be hidden.
Your letters to grandma are in envelopes, not because your correspondence with her is "interesting" in any real sense, but because letters in envelopes are the default. It's a social expectation, in other words, and we need to import that expectation to the Internet to the greatest extent possible, because the Internet is the new mail system, information hub, and everything else.
Plus, I really don't want to give my ISP any foothold to insert advertising.