A multi-community would be all communities with a certain name, across all instances. It would prevent powermods from being a problem on Lemmy. i think it should be notated with m/<insert name here>, just like communities but with m instead of c.
The answer to such questions is almost always "Because someone hasn't done it yet".
As others have mentioned, this platform has grown drastically over the past month and so the devs are somewhat preoccupied with what they've prioritised for the platform as a whole. This feature though is on the radar, as others have said.
A quick fix that I actually think would help be just in the UI, where the user can group the communities they're subscribed to into what ever groups they like, so that it becomes easier to browse through communities individually by topic.
