Why? It's a historical artifact from the early days of the internet. It originated with the HTML 1.2 draft (1993) and was integrated with the HTML 2.0 specification. It's purpose was to indicate the title of the document--as simple as that.
In the beginning, the purpose of HTML was to provide a markup language that could be used for books and documents--not our modern understanding of web pages.
Back in those days, they had this great tag called "Title"
<TITLE> Moby Dick</TITLE>
The purpose of the TITLE tag was to declare the title of the document that you were viewing.
However, when you provided a link, you could add a title to that as well, so that the user knew what the next document was:
<A TITLE="Moby Dick" HREF="...">Next Book</A>
After that point, the title attribute moved to the other tags. Later, when you added a "title" to a picture, the browsers chose not to display that immediately on the webpage, but to add it to the tooltip. As it moved to other elements, it just became the tooltip.