There are thousands of subreddits, sure, but still you end up with an identifiable overarching "reddit" culture. There are subreddits that don't share this, due to strong moderation, small size, or niche/polarizing subject matter. But in most subreddits, mentioning a broken limb, for example, will inevitably generate a low-effort allusion to a certain gross story that is prominent in the collective memory of the site. A positive reference to feminism will likely spur someone to try to start a flame war. Even if you unsubscribe from the defaults, the site-wide zeitgeist stills filter down and influences conversations across individual sub-communities.