Not generally useful to show this by default, because nowadays most pages are dynamically generated and although it's technically easy to implement, the last modified header is typically not set to $now.
Well, if Browsers would show it more prominently, there were more motivation to think about it for developers.
But Browsers are bad HTTP clients. Think about bad user experience with file uploads (no built in progress report!?), HTTP auth (not showing status, no logout etc.)
In Chrome you can F12 and go to "Network" tab and then refresh the page. Choose the first file in the list (that's the HTML itself) and you will find "Response Headers" in the "Headers" panel, which includes Last-Modified. It's a bit deep, which makes sense as it's rarely useful.
Not generally useful to show this by default, because nowadays most pages are dynamically generated and although it's technically easy to implement, the last modified header is typically not set to $now.