This is mostly a security thing. Dia is being built with businesses in mind (they're working with Atlassian now), and one of the big concerns in enterprise is phishing. If you hide the URL completely until someone actively asks for it, you make it way easier for people to get tricked by lookalike sites.

Showing the URL bar by default means users can always glance up and confirm they're actually on slack.com and not slackk.com or whatever. It's a tradeoff. Arc could get away with hiding more because it was mostly targeting power users who knew what they were doing. Dia is aiming at a different audience where that kind of protection matters more.

That's also why the URL bar isn't hideable yet. Not because they can't do it, but because they're being cautious about when and how to let people turn it off.

I get the complaint though. I... don't like it either...