Matrix is terrible for hosting communities compared to Matrix. Matrix is only good if you are okay having a single group chat for everyone to put everything into. A single group chat does not scale, so it's better to use a platform like Discord.
At least in the mobile app I didn't even know this existed. To find this directory of rooms you need to open up the spaces drawer. The you need to open a menu for the space by clicking on a stack of three vertical dots. I thought this would have things like settings or something in it, but no this is the way to navigate to important information. Then you need to click on explore rooms. You now can see a list of rooms you can join. You can now join a room with a tap. Then you have to open the room with a tap. This is extremely convoluted and hidden.
Meanwhile Discord doesn't need this whole process since you don't need to manually join rooms yourself.
However, don't see a problem. Why does it matter if they join each channel? you don't need to read a channel in Discord as well despite you are auto joined
Higher friction to visit other channels means that it will mean less people will visit other channels. I've been in a Nix / NixOS room for a while and I had no clue I was in a space and that space had other rooms until you commented that it existed. I never had this issue with discord. It was obvious there were other channels the first time I used it.
sounds like a temporary issue though: first only rooms existed, now Spaces exist, but the room directory still exposes rooms instead of Spaces