There was a category for SARS, the Concorde Crash, the Christmas Tsunami, the London Bombs, within 24 hours of the events happening. That isn't a stale directory.
There are pages on the web that haven't been edited for a decade; because the data that was true then is still true now. The ODP lists hundreds of thousands of such sites.
Any write-up of an historical event remains valid forever, unless found to be an untrue, incomplete, or inaccurate account. Those can be added any time.
Editors log in, then choose what action to do today, read forums, look at editor applications, clean up spam in the suggestion pile, work on update requests, redirect wrongly submitted suggestions, search Google or news sites for things to add, review stuff in the suggestion pile, etc. Part of that choice is also in which category to actually work on: and for each editor that action is random within their permissions list, but highly likely to be influenced by "in which category can I make a difference?".
In a remote niche topic with two listings, adding another two sites doubles the sum of human knowledge. In a spam laden pile of hundreds of suggestions, in a category where dozens of sites are already listed, the casual surfer is unlikely to notice an extra ten sites being listed, and the amount of work needed to find ten such listable sites would exceeed most editor's patience - so you'll usually find them taking the former choice when they decide "where shall I edit today?".