Frames were vaguely useful when you had a static web site, to avoid repeating navigation menu in all pages, for example. It also reduced the overall size of a page.
Both these arguments are obsolete now: sites don't hesitate to serve fat pages, and most of them are dynamically built so including such navigational parts (or status, etc.) has no problem.
The "why" part is well answered above, partly by your own question (you hit a limitation, although it can be overridden with a bit of JS).
