Because this "group" can't be open in a single separate page to see all the Forums included it. Right?
You're right, there's no way to open the list of forums inside of such "group" - but honestly, I didn't get the idea of why you would like to open it that way.
Or there is a way to do it?
There could be a way to code some add-on/extension/hack; but I'm not sure how efficient and expensive in development it would be.
Is there can not be the Groups of Forums, can the Topics have some connection with each other? Maybe to show on the top of the page of one's topic - links to the topics with similar subjects? It could be a kind of solution for this problem.
For some forums, I have coded an add-on called "Similar Topics" - the list is displayed beneath of the primary topic and it tries to list topics of the similar subjects (based on a special mySQL request) - this is the only thing I could suggest for some small fee for your purpose, too.