The decomp.me Discord server invite link can be readily found on that website and in the README of its GitHub repository. It's the closest thing to a central hub of the decompilation community. You can find some invite links in its chat history by searching for "discord.gg" (including the servers listed below).
Some of these Discord servers have a #other-servers or #related-servers channel with tons of invite links to other similar Discord servers. In particular, these servers have those channels:
- PS1/PS2 Decompilation
- GC/Wii Decompilation
That way, you should be able to find dozens and dozens of Discord servers on that topic. There are still many more out there (I've joined at least six others that aren't directly reachable from the invite links inside the servers I've mentioned).
There are also other Discord servers about reverse-engineering that can contain discussions about decompilation techniques or projects.
Second, because while the biggest decompilation Discord servers have effective moderation team and processes, the vast majority are just a server for a project from one or a couple of persons with <100 members joined. Such small servers don't have round-the-clock moderation or customized settings.
Third, because I regularly see phishing crypto spam posted in the smaller servers. Spreading invite links publicly carelessly increases the odds that these scammers find them and spam them.
There are also practical concerns. An invite link that needs to be deleted (for example due to spam abuse) means that it will no longer work. Updating all the places where that old link appeared to the new one can be impractical or impossible.