You're almost not wrong, but I think what you're discounting is how much power a lot of email clients have. Especially the "old" ones. People were hanging out on mailing lists before the web existed, so there's a lot of tooling in there around filtering, tagging, flagging, etc.

Remember flags? That feature of mail clients that's like "why would I use this?", or smart folders, that feature of mail clients that allows you to use a pre-written and saved search filter and browse it like a folder? These were written at a time when the email client was the social communication interface.

And if something in there should be insufficient, you can always write a script or something that interfaces with email as an API of sorts.

While it's true that a dedicated tool could be good, in a sense the email client is a dedicated tool for this, and importantly it's one that I control on the client side to do anything I need it to, regardless of whether or not anyone else on earth needs it to do this. My email client serves me.

Quick addendum before people come for me: I claimed email was "the" social communication tool. Yeah yeah IRC gets a say here, but we can all agree it's different. And then also newsgroups, but I don't want to open that can of worms. Just know that you've been named.