Well, mainly because this is pretty far along, and making it more convenient in a meaningful way would take some significant time and effort. For example, prompting all users at least once when they visit to fill something out. Maybe a weekly nudge/reminder.
What you're saying about Favorite Movies is exactly where I was going with this.
At some point it's worth sitting down and hammering out a decent formula wherein we combine Favorite Movies lists and combine those with submitted lists (probably giving the latter a bit more weight). Even that involves a lot of work, though; I've looked at the unfiltered list of most-mentioned movies in the Favorite Movies list, and it's terrible because it includes lots of movies (like,
300) that are popular among 14-year-olds who stop by, add their favorites, post a few times and then never come back.
The compiled list gets a bit better if we require a minimum number of posts, but it's still a little sketchy.
So, mainly it's about finding the time to do the extra work on my end and making decisions that everyone can live with, though the latter is probably impossible. Might have to wield a little dictatorial decision-making power in regards to the specifics of any formula.