Eh, it's entirely logical.

Let me explain:

• The area that most indi developers underperform at is.... Marketing. Many of them put their heart and soul into a game and then completely fail to gather a community or audience that is interested in such a game.
• so what is one of the easiest ways that a developer who has effectively no social media skills can gather a community of interested individuals to talk about provide feedback and announce changes to?
• most of the target audiences for most games are going to be on social media
• You expect to chat with your community and so do your users that care
• Discord is a well-established platform, with a large ecosystem and many integrations meant to target games
• The developer chooses Discord as it is effectively the best option for their situation and available resources

This makes perfect sense. I don't see why you should be outraged about something of this sort.

That said:

If you don't want to join the Discord, you don't have to.