Tooltips. The kind of thing people associate with the HTML title attribute. And, apparently, the alt attribute due to some nice implementations of that. (Netscape Navigator 4 and Internet Explorer 4 started this tradition as far as I know.) Now the alt attribute is for alternative content, not tooltips. Another way to look at this is taking the object element. Semantically, the following two are more or less equivalent:
<object data="image">TEST</object>
<img src="image" alt="TEST">
Do you expect both to show a “tooltip?” One? Or a more sane thing: none of the above? If image could not be loaded, would you still expect a “tooltip” when hovering the content? Here are the answers: “no,” “no,” “yes” and “no.”