Forums are not like a directory structure - there are four levels at most - categories, forums, topics, posts. Topics are comprised of posts. Forums are containers for topics. Categories are containers for forums.
When you click on a forum, you expect to see topics.
When you click on a category, you expect to see forums.
When you click on a forum and get another list of forums, are you in a category or a forum? Will you scroll down and discover topics, or will you assume that this is another category with a confusing UI and enter a subforum, never realizing that there are topics you can participate in?
But what if the subforums aren't actually listed in the forum? Where would they be displayed? The forum summary seems like a logical choice. But, many users bookmark individual forums, rarely venturing out to the forum summary page. Don't forget about favorites mode, or collapsed categories either.
And even then, how should subforums be represented on the forum summary itself? Do we just list them under the description for the parent forum, or do we create another nesting level and list all of the details? That makes the parent forum even more of a category instead of a forum...
This is not a trivial matter. This isn't a case where we're going to just jump on the bandwagon and do it like everyone else, because we don't feel we'd be doing anyone a favor. We don't feel the UI is right.
Also, keep in mind that while subforums are a frequent request, they're actually only barely inside the top 10 in terms of the number of unique individuals that have asked for them. Yes, we do keep track of these things. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />