There's definitely an issue with notifications in things like Slack/Discord/Teams, but it's a symptom of a larger problem software where everything is competing for attention. Social media apps do it because it's part of their business model and built in to the addictive-by-design need to get eyes on ads.

Chat systems aren't (yet?) dependent on ad revenue, so they don't need to copy the behavior of social media, but, possibly because they use the same operating-system level notification APIs, that's how they act.