You can do this in CSS with a dimension called "vh" and "vw" (viewport height and viewport width). If you're using JS, you can use "window.innerHeight" and a few other methods.

The problem is that in virtually every browser, this screen space is respected as the space that belongs to the developer. It would be extremely inconvenient if the "vh" value didn't take into account the browsers own UI. Because then you'd need to adjust your style value to account for that height. Well, iOS Safari does exactly that, and it's been a major source of frustration for web developers for years.

It's hostile to users because it makes developing for the mobile web fairly difficult, and thus degrades the experience. I honestly don't know why they do it, but my more cynical side says that they don't mind sacrificing UX on the mobile web because it competes with their app store.