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.