I could be wrong, but I think there are two main reasons for why there are so many different Godot communities.
The first reason is that people have preferred social media sites where they like to post, so they created Godot communities on these sites so they can discuss Godot related material on their social media platform of choice. There are quite a few people who are members of more than one community, likely because each community on each platform is slightly different.
The second reason, if I remember correctly, is that there wasn't very many officially linked Godot groups for quite awhile. This lead to many creating their own groups, perhaps not knowing that there was already a Godot group on that social media platform. I know this is what more or less happened with Discord.
The official Godot website has helped bring more awareness of the groups available through the "community" tab, which I think has helped reduce duplicate groups on the same social media platform.
I think it has gotten easier to find Godot communities now than it was before. From what I have seen of other game engines, this sort of fragmentation across multiple social media sites is common, though with Godot it has been a tad more pronounced due to how rapidly the community has grown in the last couple years.
I do think it can be confusing though. While there is a lot of overlap between communities, each one has unique aspects that may not be obvious at a glance. For example, here on the forums we do not have very many members who are experienced with the C++ side of the engine, so C++ related discussions generally do not get as much participation as other topics simply because there are not as many C++ developers.
