Why doesn't that work for main boards? That can be useful especially for those who are interested in statistics.
Are you serious? It is right on the main boards
Not only for those who care about statistics, everyone can see those numbers to the right of the main boards. Since they were already displayed there, they didn't need to design an extra html anchor to display. Basically, the operation of the forum may be defects due to users, but the structure of the forum is in the most complete and optimal state.
Example for the Marketplace board:
The whole board got 398055 topics already made but if you sum up all of its child board topics you'll only get 391841 so meaning the mainboard "Marketplace" has already made 6214 topics (this is the topic made as the time of posting this reply it may change any moment e.g. new posts, deleted topics or moved topics to other boards).
This is not affiliated with what OP is talking about