Ask HN: Why are so many OSS communities on Discord?

I'm honestly tired of seeing OSS tools/products like Gitea rely on centralized, proprietary tools like Discord and Slack for their communities. I could understand less technical communities staying on Discord, but for OSS tools - especially those that focus on self-hosted like Gitea! - to not run their own chat server seems weird. Why is this the case? Is self-hosting just too inconvenient/expensive/pointless?