www.example.com and example.com might be two different websites.
If you want the content at www.example.com and you go to example.com, then may not get the content that you wanted. One might be a shop selling shoes and the other might be a shop selling hats.
If you want a hat and you go to the one selling shoes then you're going to be disappointed.
'www.' doesn't 'mean was HTTP accessible'. You can make any system HTTP accessible, or not if you want to.
It doesn't matter how you or I set up our websites, or whether you or I think it's a good idea to have different websites at these addresses, it matters how other people set up their websites and if they choose to do this or not. Some do! Therefore the user needs the information.
You could send different websites based on what port the request comes from. But doing so would be bad and wrong, and showing the user the outgoing port is definitely not necessary.
But that's you controlling that, as the person running the website.
When it's the user controlling it, by setting which domain to visit, that's in their control, so they need to see it in their UI.
If you're going to reduce to the absurdity of 'anyone could serve anything from anywhere' then why show a domain at all? ebay.com could serve me the content of google.com, so let's not bother with domain names? Is that your argument?
I'm not making an argument about what should be shown right now, I just think your argument, based on what "might be different", is pretty flawed.
But there's no standard that requires this, that I'm aware of.
Chrome are acting like there is, but I believe there isn't.
The specific of how you've decided to run your site or serve your pages is irrelevant. You may serve a page depending on if the http request came over port 80 or some other port or via http or https or at this time or that time or other.