Many organization and companies don't want outsiders to see what is happening due to privacy and security issues. Anyway, there is a PR opened for Zulip which allows people to see the content of public streams without logging in. It's up to the administrators to decide which streams would be public to users which are not logged in.
Interesting to know, thanks for mentioning :- ) B.t.w. I like Zulip's threads.
A thought: What if Zulip, during installation, let the admin choose if things should be public or hidden-behind-login, by default? E.g. useful for open source projects
Here're open source Slack alternatives for public communities, no login screens: https://spectrum.chat and https://www.talkyard.io (I'm developing it).
(Rocket Chat & Mattermost & Zulip are other alternatives, all require login to see what's there? )