Tooltips show the full title of the tab, which is useful if the title is long, the tabs are small because there are a lot of them, or it's a pinned tab

The page title isn't necessarily visible on the web page that sets the title.

Clicking is not always a simple task.

I shouldn't have to leave my current page just to figure out what another tab is.

Again, just because you feel something is useless or easily avoided doesn't mean that all internet users feel the same.