There’s absolute mass quantities, as Beldar the Conehead might say.
It’s easy enough to guess that plenty of people just grabbed a community name in case they might find it useful one day, but I’m guessing plenty of others legitimately started up a community, put some effort in to it, then ultimately got discouraged and abandoned it. A big part of that likely due to not being able to attract many subscribers and contributors.
Personally what I’ve found is that if you really want a community to grow, you need to seed it with content on a regular basis; preferably daily. Posting bots are probably a good way to help with that, altho if the sub looks like it’s little more than bot posts, I don’t think users will be inclined to post or comment much.
What I haven’t quite figured out myself is how to incline users to post on their own, but hopefully with time that issue will kind of resolve itself due to sheer user count.