Aren't those subreddits default? Which would explain why so many people joined.

I don't generally "join" any subreddits anymore. I create a multireddit and add subs to the multireddit. My joined subs is really small and are the subs I monitor regularly so I see them from the main page. Multireddits let me "subscribe" to less frequently visited subs without actually joining or being forced to see them.

For instance, I have one for books, one for music, one for finance related subs, one for programming subs, one for humor, etc. Almost none of those contain subs that I'm subscribed to.